34404 create rev from json

Allow creating CDTDocumentRevision objects for use with remote data sources

Create factory method which takes an NSDictionary and document URL.
- Reject dictionary if it contains any _prefixed values which are not couchdb properties
- Create any attachments as CDTRemoteAttachments defined in _attachments

The bugs likely to occur from this are using the API with NSDictionary objects containing incomplete data.

Did you think about using https://developer.apple.com/library/mac/documentation/Foundation/Reference/NSURLSession_class/Introduction/Introduction.html#//apple_ref/occ/instm/NSURLSession/dataTaskWithURL:completionHandler: instead to make this call asynchronous?