Adds delegations to content and essence models.

- restricted?
- public?
- trashed?

are delegated accordingly.
Register each essence as has_many association.

Each class that uses `acts_as_essence` now creates a has_many association on Page and Element classes.

i.e: EssenceText creates a essence_texts association
Fix api contents_controller spec

Because of the delegation we need to create the object, not stubbing.
Merge pull request #688 from magiclabs/feature/essence-associations

Register each essence as has_many association.