Add an “animate alongside” hook

[wtmoose]

[wtmoose]

Cc @mergesort

[wtmoose]

@mergesort

Check the work/animateAlongside branch. I've also attached a quick mockup of a matching tab bar animation. But there's a complication: the tab bar's barTintColor property isn't an animatable. As a workaround, I used UIView.transitionWithView(). The problem with this is I had trouble animating the message view's background color in sync. It seems the easing curves don't quite match. Maybe there's an option I've missed? My thinking is you'd need to drop into Core Animation and snag the timing curve from the transition animation in order to animation the message view's background color.

What do you think?

SwiftMessagesTest.zip