Add ObserverType to Observer, for constrained extensions.
Add ObserverType to Observer, for constrained extensions.

andersio/ReactiveCocoa,sandyway/ReactiveCocoa,calebd/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,victorlin/ReactiveCocoa,walkingsmarts/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,calebd/ReactiveCocoa,andersio/ReactiveCocoa,smilypeda/ReactiveCocoa,sandyway/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,ohwutup/ReactiveCocoa,ikesyo/ReactiveCocoa,smilypeda/ReactiveCocoa,sandyway/ReactiveCocoa,victorlin/ReactiveCocoa,smilypeda/ReactiveCocoa,calebd/ReactiveCocoa,ohwutup/ReactiveCocoa,victorlin/ReactiveCocoa,andersio/ReactiveCocoa,walkingsmarts/ReactiveCocoa,walkingsmarts/ReactiveCocoa,ohwutup/ReactiveCocoa,ikesyo/ReactiveCocoa