Remove unused api.future functions

 - api.future.toSuccess and api.future.toFailure
Rename Future.success and Future.failure to match Promise functions for consistency
Add api.future.isCompleted for internal use
Revert removal of api.future.toSuccess and api.future.toFailure

- used in api.future.tryf but missed because of no spec!
Update ajax module for Future.resolve and Future.reject