Add animating container views.

This PR adds two container `NSView` subclasses, that use Auto-Layout to (optionally) animate the swapping of their content views.

You can see an example of how they look in GitHub for Mac.

[RBLExpandingContainerView](http://f.cl.ly/items/1a1f3d382s3P220i0W1j/Animating.mov)

[RBLSlidingContainerView](http://f.cl.ly/items/3t2Y2X2h3n0u2L2f0D0O/Sliding.mov)