Additions for guns

Adds chamber to mag-type guns.
 Ammo now loading into the chamber.
 Adds some new sprites for ammo mags.
Renamed load_into_chamber() to process_chambered()
![45](https://cloud.githubusercontent.com/assets/5725070/4320845/e87a3bac-3f3a-11e4-9718-4934c2af3f4e.png)
![smg](https://cloud.githubusercontent.com/assets/5725070/4320847/edb046ac-3f3a-11e4-9929-0534835b263b.png)
![uzi](https://cloud.githubusercontent.com/assets/5725070/4320848/f1d395fe-3f3a-11e4-9963-5d139a27ef9f.png)
Pre-tested

Could you update this to 8, since that's the new mag size? It's useful for purposes of consistency, though magazine guns don't use max_shells.