[WIP] Ore generation

This is my first contribution to the glowstone project, so I would appreciate some feedback.

Todo:
- [x] Make ores spawn on the right y-coords
- [ ] Make ores spawn in veins
- [ ] Add xp drops
- [ ] Let ores spawn rare on certain levels, and common on certain levels

References
http://minecraft.gamepedia.com/Ore
Kill it with fire! [Tab character..]