Add animating container views.

This PR adds two container `NSView` classes, 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/0G0Q1r013g461i0A0B1p/Animating.mov)

[RBLSlidingContainerView](http://f.cl.ly/items/3k1e332o1M2A3Q1j3P3f/Sliding.mov)