Cargo Trains

![train](https://cloud.githubusercontent.com/assets/6357112/3685427/e12742c0-130a-11e4-8075-e6b7e4abfe96.png)

- Adds sprites for cargo trains (thanks to Bishop)
- Adds trains to the cargo orderables (30 for each engine/tug, 20 for each trailer)
- Adds a single train on the map in cargo bay (1 engine, 3 trailers) ~in a separate commit, so if things go bad they can be easily removed from the map
- Fixes a couple of bugs (details in the commits)