[Discussion] ReactiveCocoa 2.1.5, moved iOS6 stuff to dedicated subspec

I moved all stuff that needs iOS 6 to separate subspec. Actually I should have done this when UICollectionView category was introduced.

I didn't push it right to @CocoaPods/Specs because this might break existing installations (i.e. `pod update` may lead to compilation failure).

/cc @tonyarnold