Custom Background

This adds the ability to set a custom background.

To use a custom background, just add a `background.png` image in your roms directory. Kart will see it and use it. If you don’t provide one, the default included background will be used instead.

The background image will first be measured, any background under `600px` wide will be tiled, anything over `600px` will be stretched across the background.

This allows you to both use a tiled image or a full screen image for the background.

![animated-2014-08-13_15h-18m-06s](https://cloud.githubusercontent.com/assets/260/3911145/97514384-231e-11e4-8e22-3b16d658ea0b.gif)