Extract methods from not embeddable objects into Documents and clean Model class.

Update tests
Add Embedd Errors
[WIP] embedded objects
Persist embedded objects
Update documentation and comments
Remove `save` and `save!` methods from embedded objects.

Only Documents can be saved
Add tests for embedded object persistence
Use the new `get_many` method for retrieve multiple keys from this bucket. 

This is way too much better than iterate over the keys and do multiple GETs to the database.
Embedded objects should not respond to `new?` or `delete`. 

Those methods are only valid for Documents.
Define equality for embedded objects and update equality tests
Use `model` instead of `self.class`
Fix persistence for embedded objects not defined
Support embed a collection of objects
Merge pull request #6 from emancu/embedded_objects

Embedded objects