Switched between tilesets with each level.

In the gamedata class, an array of strings lists all available tilesets. At the moment, there areonly  three tilesets (stoney, snowy and dirty), but we should eventually add two more tilesets so there is one tileset per level. The levels run through the tilesets in the order that they are listed.

![image](https://cloud.githubusercontent.com/assets/1202938/4763697/ac076158-5b15-11e4-8297-72bb751ac7eb.png)

Oh, and I also added some devmode stuff.