WIP Inventory slot system [Don't merge yet]

This is the start of an inventory system I'm creating. Here's a cool image to show you what I've accomplished so far:
![screen shot 2013-12-16 at 2 19 05 am](https://f.cloud.github.com/assets/711973/1752726/b90f1210-6629-11e3-884b-a8bc2fde54b7.JPG)
I've also posted [a link to the same image](http://cl.ly/image/2I0K0i1t240m) on the test server, provided nobody changes it.

Keys 1-9 select the inventory slots, you can also scroll or press E to cycle. The selected slot determines which block you will place.

TODO: Allow item changing, make items limited, display count, background image, etc.

@fogleman: You probably don't want to merge this until it's finished.