Cherry picked from aborted 3.0. - Replace -didSubscribe…: with a disposable property

Conflicts:
	ReactiveCocoaFramework/ReactiveCocoa/RACChannel.m
	ReactiveCocoaFramework/ReactiveCocoa/RACDynamicSignal.m
	ReactiveCocoaFramework/ReactiveCocoa/RACPassthroughSubscriber.h
	ReactiveCocoaFramework/ReactiveCocoa/RACPassthroughSubscriber.m
	ReactiveCocoaFramework/ReactiveCocoa/RACSubject.m
	ReactiveCocoaFramework/ReactiveCocoa/RACSubscriber.h
	ReactiveCocoaFramework/ReactiveCocoa/RACSubscriber.m
Initial implementation of RACKVOProxy per discussion in https://github.com/ReactiveCocoa/ReactiveCocoa/issues/1122
Integrate RACKVOProxy into RACKVOTrampoline; adjust how we generate context parameters to avoid collisions in the proxy table
Increase speed of +combineLatest: by 60% by converting to a non-recursive implemention.
Temporary hack fix to prevent crash correlated to race condition between addObserver on low priority thread followed by immediate removeObserver on main (from @epatey)
Failing unit test to expose thread-unsafe implementation of -[RACSerialDisposable disposable].
Reimplement RACSerialDisposable with OSSpinLock rather than OSAtomic in order to fix -disposable method to be thread safe.
Add [UIAlertView rac_willDismissSignal]. This is required when events are required if the alert is dismissed programatically. rac_buttonClickedSignal does not fire an event in that situation.
Add -[RACSignal deliverOnMainThread]