Generate add/remove methods for "many" relations (fixes #487)

Methods will have the singular form of the field name. The conversion from the plural form needs improvement (ideally something like StringUtil.php from Symfony's PropertyAccess component).

Related ORM issues and commits:

DDC-1497: doctrine/doctrine2@1f2ce21b5613d0cc2500946afc9553e514314c34
DDC-1907: doctrine/doctrine2@5cdb0ae8beaf29d924f0362341d4965cca0f1dbe