Provide the EmailValidator entry point to send the API requests

So far we just validate the provided arguments.
Waiting for @Nyholm confirmation about the best way to proceed.
Please refer to
https://github.com/mailgun/mailgun-php/issues/191#issuecomment-350133021

src/Mailgun/Api/EmailValidation.php

@@ -0,0 +1,131 @@
+<?php
+
+/*
+ * Copyright (C) 2013 Mailgun
+ *
+ * This software may be modified and distributed under the terms
+ * of the MIT license. See the LICENSE file for details.
+ */
+
+namespace Mailgun\Api;
+
+use Mailgun\Assert;
+use Mailgun\Exception\InvalidArgumentException;
+
+/**
+ * @link https://documentation.mailgun.com/en/latest/api-email-validation.html
+ *
+ * @author David Garcia <me@davidgarcia.cat>
+ */
+class EmailValidation
+{
+    /**
+     * Given an arbitrary address, validates address based off defined checks.
+     *
+     * @param string     $address             An email address to validate. Maximum: 512 characters.
+     * @param bool|false $mailboxVerification If set to true, a mailbox verification check will be performed
+     *                                        against the address. The default is False.
+     *
+     * @throws InvalidArgumentException Thrown when validation returns an error
+     */
+    public function publicValidate($address, $mailboxVerification = false)
+    {
+        // Validates the email address.
+        Assert::email($address);
+
+        // Validates the mailbox verification.
+        Assert::boolean($mailboxVerification);
+    }
+
+    /**
+     * Parses a delimiter-separated list of email addresses into two lists: parsed addresses and unparsable portions.
+     *
+     * The parsed addresses are a list of addresses that are syntactically valid
+     * (and optionally pass DNS and ESP specific grammar checks).
+     *
+     * The unparsable list is a list of character sequences that could not be parsed
+     * (or optionally failed DNS or ESP specific grammar checks).
+     *
+     * Delimiter characters are comma (,) and semicolon (;).
+     *
+     * @param string     $addresses  A delimiter separated list of addresses. Maximum: 8000 characters.
+     * @param bool|false $syntaxOnly Perform only syntax checks or DNS and ESP specific validation as well.
+     *                               The default is True.
+     *
+     * @throws InvalidArgumentException Thrown when validation returns an error
+     */
+    public function publicParse($addresses, $syntaxOnly = true)
+    {
+        // Validates the email addresses.
+        Assert::stringNotEmpty($addresses);
+        Assert::minLength($addresses, 3);
+        Assert::maxLength($addresses, 8000);
+
+        $arrayOfAddresses = preg_split('/;|,/', $addresses);
+
+        foreach ($arrayOfAddresses as $singleAddress) {
+            // Validates the email address.
+            Assert::email($singleAddress);
+        }
+
+        // Validates the Syntax Only verification.
+        Assert::boolean($syntaxOnly);
+    }
+
+    /**
+     * Addresses are validated based off defined checks.
+     *
+     * This operation is only accessible with the private API key and not subject to the daily usage limits.
+     *
+     * @param string     $address             An email address to validate. Maximum: 512 characters.
+     * @param bool|false $mailboxVerification If set to true, a mailbox verification check will be performed
+     *                                        against the address. The default is False.
+     *
+     * @throws InvalidArgumentException Thrown when validation returns an error
+     */
+    public function privateValidate($address, $mailboxVerification = false)
+    {
+        // Validates the email address.
+        Assert::email($address);
+
+        // Validates the mailbox verification.
+        Assert::boolean($mailboxVerification);
+    }
+
+    /**
+     * Parses a delimiter-separated list of email addresses into two lists: parsed addresses and unparsable portions.
+     *
+     * The parsed addresses are a list of addresses that are syntactically valid
+     * (and optionally pass DNS and ESP specific grammar checks).
+     *
+     * The unparsable list is a list of character sequences that could not be parsed
+     * (or optionally failed DNS or ESP specific grammar checks).
+     *
+     * Delimiter characters are comma (,) and semicolon (;).
+     *
+     * This operation is only accessible with the private API key and not subject to the daily usage limits.
+     *
+     * @param string     $addresses  A delimiter separated list of addresses. Maximum: 8000 characters.
+     * @param bool|false $syntaxOnly Perform only syntax checks or DNS and ESP specific validation as well.
+     *                               The default is True.
+     *
+     * @throws InvalidArgumentException Thrown when validation returns an error
+     */
+    public function privateParse($addresses, $syntaxOnly = true)
+    {
+        // Validates the email addresses.
+        Assert::stringNotEmpty($addresses);
+        Assert::minLength($addresses, 3);
+        Assert::maxLength($addresses, 8000);
+
+        $arrayOfAddresses = preg_split('/;|,/', $addresses);
+
+        foreach ($arrayOfAddresses as $singleAddress) {
+            // Validates the email address.
+            Assert::email($singleAddress);
+        }
+
+        // Validates the Syntax Only verification.
+        Assert::boolean($syntaxOnly);
+    }
+}