Adds catwalks

**Adds catwalks!**

Child of lattices.
Basically lattices you can lay cable on, and much easier to make than plating because it just requires more rods rather than a floor tile.

Featuring:
Bad sprites ported from bay!

![ss 2014-12-26 at 06 33 04](https://cloud.githubusercontent.com/assets/7247232/5560190/0129c26c-8d2e-11e4-81bc-b31acdbf38bd.png)

They are more or less a replacement for having all those ugly airless plating tiles in space around the station.