[WIP] ObjectManager: add base class and interface

Add base class and interface for the several object manipulation classes

Children classes would have to manage the specific objects for each (ex: ContentTypeGroup, Users, ContentType, ...)

This is intended to be the higher class for the creation of objects on "Given" steps and load/read/count for some "Then" seps

For example: 
[ContentTypeGroupManager](https://github.com/MLoureiro/ezpublish-community/compare/bdd_add_contenttypegroup_manager)