[WIP] Add RBBRubberbandAnimation

This adds `RBBRubberbandAnimation`, which is an alternative to `RBBSpringAnimation` and `CASpringAnimation` that is optimized for `CGPoint` animations and correctly handles the `velocity` paramter, even if ∆x or ∆y are 0.

<p align=center>
<img src="https://cloud.githubusercontent.com/assets/212465/2625279/c4e3ecba-bd8a-11e3-8a2a-f64fed4a30d7.gif">
</p>

```objc
RBBRubberbandAnimation *rubberband = [RBBRubberbandAnimation animationWithKeyPath:@"position"];

rubberband.from = CGPointMake(100, 100);
rubberband.to = CGPointMake(100, 100);
rubberband.mass = 1;
rubberband.damping = 10;
rubberband.stiffness = 100;
rubberband.velocity = CGVectorMake(400, 0);
```

`RBBSpringAnimation` will remain as an alternative to CASpringAnimation.