Added Query Hints support to Entity form type that use query builder

Adds an optional "query_hints" array to be passed to "entity" form types, to be used when generating the Query object to retrieve objects from storage.

I've added a basic test, that just validates the scenario where the input type for the "query_hints" is incorrect. I wanted to add a test to validate the working scenario, but my phpunit/doctrine knowledge only goes so far, so if anyone could give me a help with it, I'd really appreciate.