-animatedSignals for getting a signal of animation signals

This replaces both -doAnimationCompleted: (which was too stateful) and
-completeAfterAnimations (which is subsumed by the new semantics).
Remove animation tests from iOS target

They won't work from logic tests (application tests are needed) because
they now depend on animation completion behaviors.
Update ResizingWindow demo
Clarify that only iOS supports interruptible animation
Fix animation based on nameLabel width

Probably broke in iOS 7.
Use `alpha` property instead of `rcl_alphaValue`

Since animation interruption only makes sense on iOS, reflect that in the examples.
Merge pull request #92 from ReactiveCocoa/inner-animation-signals

Replace -doAnimationCompleted: and -completeAfterAnimations with signals of signals