Remove deprecated APIs

- Remove deprecated APIs
- Remove legacy API mode to document update internals
- Remove usage of deleted APIs from Sample
- Changed the description property in CDTTodo to taskDescription to fix
  warning about property redeclaration (description is declared in
  NSObject)
- Changed test file names, removing suffixes indicting new API tests
- Updated CHANGELOG.md
- Created API migration document
Merge pull request #64 from cloudant/35676_remove_deprecated_apis

35676 remove deprecated apis
Fix merge issues
Create factory method to create document revision from downloaded JSON data.
Whitespace and improve doucmentation around accepted json data
Allow all keys that are valid _ prefexied keys for couchdb/cloudant json
and add test to make sure they are all allowed.
Whitespacing and added test that attempts to create revision with invalid JSON, JSON string is incomplete
Implemented CDTRemoteAttachment

Changed the factory method of CDTDocumentRevision to ask for the URL of the document
Creates attachments from the json included from the get request. Downloads the data
when the data is asked for, or parses the Base64 encoded attachment data included with
the document get request if present.
Fix compile errors and change expected behaviours
Resolve compile errors a result of rebasing
Changed how the test creates a conflict to be resolved, due to validations a revision cannot be create with a nil body or a nil rev Id
Change factory to take NSDictionary instead of NSData to remove overheads for document updates
Changes for issues pointed out in comments
Update comment for remote attachment factory method
Change name of CDTRemoteAttachment to CDTSavedHTTPAttachment