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

Note: CDTMutableDocumentRevision does not handle attachments yet, this functionality will be added in a later PR
Make dictionary keys explicit (I guess they are "sha" and "size" but the comment doesn't make this clear)