Wall and D2K turret polish

![20140405-d2k](https://cloud.githubusercontent.com/assets/167819/2621984/f097dcbc-bc82-11e3-980a-4d274e2159a1.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.