Add a lint script

Make it easy to lint in stand-alone fashion.

Note that I'm going with a direct call to ESLint, as using `grunt eslint` leads to warnings about a circular dependency (per trace it seems to originate in istanbul?)

package.json

@@ -46,6 +46,7 @@
     },
     "scripts": {
         "test": "grunt test",
-        "prepare": "grunt build"
+        "prepare": "grunt build",
+        "lint": "eslint ./src"
     }
 }