[Update] Add "MagicalRecord/Core" and "MagicalRecord/Shorthand" subspecs

User needed MR_SHORTHAND enabled should use "MagicalRecord/Shorthand" instead.

Discussion: https://github.com/CocoaPods/Specs/commit/9e7d7aa808ed19c4e6ea1e9647cd2a7668560886#commitcomment-5290889