Implementing class methods for models - Question & Content

Implemented Question methods:
- delete (instance): logically delete the Question
- by_key: retrieve a Question by a given key
- delete_by_key: delete a Question for a given key
- add: create a new Question
Implemented Content methods
- by_text: retrieve a Content for a given text (content)

Several methods have been renamed in order to establish a new naming
convention (by_*)