Uses swipe slope to determine if you want to scroll left/right or up/down

This PR uses slope to determine the direction you're looking to swipe. The 45 degree angles are the threshold. Here's a picture to illustrate

![Screen Shot 2013-04-17 at 3 01 26 PM](https://f.cloud.github.com/assets/170299/393989/611754de-a7aa-11e2-80ed-6a70147bedd6.png)

I'm not too happy with how the code looks right now, but I can't think of a clean way that provides the same functionality. If anyone has any improvements, let me know.
The major pain is that `e.preventDefault()` should not be triggered with swiping up or down.