Create Revision from JSON

- Create factory method to create document revision from downloaded JSON
  data from CouchDB/Cloudant.

- Created CDTSavedHTTPAttachment which represents a HTTP accessable
  attachment, this can either be in CouchDB/Cloudant or a generic
  datastore.
Merge pull request #56 from cloudant/34404_create_rev_from_json

34404 create rev from json