Added a site map for the root entry point.
Allow multiple errors for a single ProcessingError.
Added jsonify_collection and deserialize_request.

Added
-----
* New function deserialize_request loads a JSON object from a request
  and raises the appropriate errors if there's a problem.
* New method for Service class to jsonify a collection of resources.
Use foreign keys and JsonMixin.

Changed
-------
* Use foreign keys for model id and owner.
Use association table for model-tag relationship.

Changed
-------
* Use an association table for the many-to-many relationship of model
  tags.
Use the JsonMixin for the names model.
Removed code that's now in JsonMixin.
Use JsonMixin to serialize responses.
Use JsonMixin to serialize responses.
Use deserialize_request and JsonMixin to serialize.
New assertions for successful responses.
Updated tests.