Improving menu robustness and bounciness.

Commit ff246a4 contains bounciness improvements. Namely, the menu now bounces when it drops from the top.
In fact, that's the behavior I expected before I first saw it in action. Just feels more natural to have it bounce when it falls down onto something. I did not remove the present bounce ("pull down" before "pull up" when being dismissed) Both are controlled by the same property.

Commit c88464b makes the menu more robust with respect to tapping the open/close button like a madman. It doesn't flicker anymore.

Cheers and keep up the good work!

PS: Might port this to CoreAnimation animations later and add some UIKit Dynamics, to have more granular control over opening/losing animations and some fluffy effects. Running short on time right now though.