Large power substations

After a couple of days I finally got around to adding the 2x2 substation, with a new "mini-boss".

Nice, big open area between the equipment (unlike the small station), but with more room for stuff to go crazy, like with the new enemy.

(Didn't want to spoil it for anyone, so it's not in the picture)
This is pretty much the layout.
![large station](https://cloud.githubusercontent.com/assets/4348451/4104123/8a2c9f52-3181-11e4-8d5b-661dfcaa549e.png)