Horizontal scrolling

I needed a way to scroll horizontally instead. I saw a previous pull request that had isHorizontal flags spread across the code, but I tried to work up something more abstract.

To implement horizontal scrolling we essentially mean variable width. Where the original list has a totalHeight property, I added a totalWidth. To trigger this though, we need to set our height to something static.

I have a boundHeight property that you'll need to set to a number that your height will be. This then lets the content flow horizontally.

Quick notes:
All existing APIs are the same, so scrollTo is still scrollTo(y) to keep it compatible. I just added a scrollToOffset(x,y)

And uh, haven't quite gotten around to tests for this. ;D But I'm at a point where it looks fine and I can work on it on my site, so I figured I'd get it out here first for you to see as well.

Thanks for all the work on list-view!