Wall and D2K turret polish

![20140405-d2k](https://cloud.githubusercontent.com/assets/167819/2621975/2b9bc36a-bc82-11e3-857b-6624b56cf481.png)

This extends RenderBuildingWall to multiple actor types, and updates adjacency when a neighbour is destroyed or sold.  The D2K turrets now integrate with neighbouring walls, and have fixed muzzle flashes and firing offsets.