Split animation behaviors into a separate library

[jspahrsummers]

RCL's animations are super amazing, but not well-known at all, and the rest of the framework scares off folks who might otherwise use them.

It's also just poor modularity: the animation and layout APIs don't really have any dependency on one another, so why should they be part of the same library?

I think we should spin that stuff off into a separate “ReactiveAnimation” framework or something.

[kastiglione]

:+1: :+1:

[alanjrogers]

Sounds Good To Me™

[jspahrsummers]

I've started this over in ReactiveAnimation. Once https://github.com/ReactiveCocoa/ReactiveAnimation/issues/1 is done, we can remove the equivalent code from RCL.