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
Is it possible to link to CDTMutableDocumentRevision, eg:

@return a @{link CDTMutableDocumentRevision}

I'm not totally sure of the cocoadoc markup, you might have to search around for this.

Also I think the signature of mutableCopy should declare that it returns a CDTMutableDocumentRevision unless there's a good reason not to (you can forward declare CDTMutableDocumentRevision if needed)