Run transitions on NSOperationQueue.mainQueue

Follows same design as Bubblewrap gem's notifications:
https://github.com/rubymotion/BubbleWrap/blob/36062f34db57d3bca1564db8f8
7b0aa1f046b823/motion/core/ns_notification_center.rb
Merge pull request #2 from JonasNielsen/transition-queues

Run transitions on NSOperationQueue.mainQueue
Change spec semantics to be more stable
Fix a spec that broke with RM's block variable changes