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