This changes the parsing grammar and validation rules to more correctly implement the current state of the GraphQL SDL proposal (facebook/graphql#90)
ref: graphql/graphl-js#1102
ref: graphql/graphql-js#1000
BREAKING CHANGE: SchemaBuilder::build() and buildAST() and constructor
removed the typedecorator, as not needed anymore as library can now resolve
union and interfaces from generated schemas.