Fix return type of count fetch request

This commit fixes the `countResultOfFetchRequest:withContext:error:`
return type so that we correctly return an NSArray with one NSNumber
value equaling the count of objects.
Add Kiwi for Unit Testing/BDD
Add empty specs for iOS and OS X specific tests
Add PFIncrementalStore to iOS and OS X Test PCH file

This commit adds PFIncrementalStore to the iOS and OS X Test PCH files
so that it doesn’t have to be included in each test file.
Add initial tests for backing store methods

This commit adds initial tests for the backing store methods.
Additionally, mock objects were added to the project to allow for
easier setup of testable objects.

Note: most tests are marked as pending and not implemented.