add new Symfony factory syntax support

This resolves #21.
I've moved to code around a bit in my own branch (to allow for a smaller "if" fork and have some specialized helper methods), see https://github.com/matthiasnoback/symfony-service-definition-validator/commit/64484c7afd86ecc4ee9b635912eed43891fcad0a I'm not sure what would be the best way to collaborate though - do you have any suggestions?