Add support for infinite horizontal scrolling

Hi, I've gone ahead and added support horizontal scrollers in the scroll-infinite build. This PR contains a few things.

### Removed PHP dependency for infinite scroll demo

I think it's cleaner no to require the server hosting the files to run PHP. That way you can serve the files from any directory with `python -m SimpleHTTPServer`  or nodejs `http-server`.

### Added support for horizontal scrolling to scroll-infinite

The code was assuming a vertical scroll. The logic is exactly the same, I just swapped some variable names with conditionals.

### Added infinite-horizontal demo

It's basically a combination of the horizontal demo & the infinite demo.

![](http://i.imgur.com/q9K0cBt.gif)