Introduce CDTMutableDocumentRevision

Created CDTMutableDocumentRevision Class
Modified CDTDatastore to add create, update and delete APIs that use Revisions.
Added basic tests for new APIs
Modified CDTDocumentRevision to add mutablecopy method

Deprecated CDTDocumentBody
Deprecated CDTDocumentRevision method documentAsDictionary
I would put this code in the datastore code. I don't think it is part of this class's responsibilities to know how to convert itself into a TD_Revision.