Add Parse-RACExtensions submodule
Fix issue in activity view.

Cherry-picked from ParsePlatform/AnyWall@0acd62f36bf3de37caa0718edd91ab9cb25ee961
Explain fork in README
Prepare PAWNewUserViewController for RAC conversion
Use RAC to control "done" button enabled state
Use RAC for performing user sign-up logic
Use RAC to validate sign-up form
Use a RACCommand for controlling sign-up form submission