SQLFilters enahancements

SQL filters in Doctrine are a bit too limited for my taste. 
This pull request makes the current connection available inside sql filter classes. 
The connection is needed to quote identifiers or values and generate sql statements for the current database. (sample use-case https://github.com/Atlantic18/DoctrineExtensions/blob/master/lib/Gedmo/SoftDeleteable/Filter/SoftDeleteableFilter.php)
Currently you can access the connection only through reflection which let's face it - is a hack.

I've also made the class depend on EntityManagerInterface instead of EntityManager.

There's a test for the new method.