Implement -webkit-overflow-scrolling: touch; in browsers that support it

[scottjehl]

Steps:

1. Add a support test for this CSS property, and add a class to the HTML element if it's supported
2. Where supported, fix page div height to the height of the screen and use this property for overflow, eliminating any need to scroll to top when changing pages.

...there's going to be a lot more involved than just these steps, but the general idea is to see how this property will improve page transitions, since it will allow us to move directly from pointA to pointB without jumping the window scroll to top.

[scottjehl]

This just landed on HEAD. It's disabled by default. Since iOS5 is still in beta, it's not common yet. Here's how jQM will look as a result whenever the upgrade comes out. http://vimeo.com/28776692