Add powerups, limit explosions to first explodable.

r: @alexcoco 

![bomberman_powerups](https://f.cloud.github.com/assets/1189716/2306850/9f880e76-a299-11e3-9d9a-4ed1e07f7825.gif)

Lots of code.

Add `cell.Cell`, which have z-layers.  Add `GameObject` that cells hold, implemented by `termbox` objects aka `TboxGameObj`. Add `Game` struct that holds the state of the game. Use probabilities to place random items, such as rocks and powerups.  Put powerups under some rocks randomly.

Tried to refactor a bit but this is ugly and a mix of procedural/functional/OO. Wtv.  Please feel free to complain about the name of variables, things that should be functions, things that shouldn't, things that should be different, etc.