Added ability to rescale used tileset at runtime using action.

These are all screenshots taken from the same instance of the game, no restart was required:

![4](https://f.cloud.github.com/assets/902211/2120446/d2a3af86-91b4-11e3-802f-707271ba660f.png)
![8](https://f.cloud.github.com/assets/902211/2120448/d65f26f0-91b4-11e3-9305-523d998e573a.png)
![16](https://f.cloud.github.com/assets/902211/2120449/d904c2a2-91b4-11e3-878a-90a60d875861.png)

Looking around for zombies:
![lookout](https://f.cloud.github.com/assets/902211/2120451/ddde3d26-91b4-11e3-861c-213165a8d301.png)