Horizontal scrollbar support.

I'd been looking for some tools to make a horizontal parallax app and really liked your approach. I noticed it could only use the vertical scroll bar though so I duplicated some code and added support for a horizontal flag. It currently does not seem able to force the width in chrome and mobile safari but it works great on Firefox and IE. It would be great if you had any input. 

I apologize for the mess auto format made, I use it to fix missing braces and parenthesis.