Actually isometric terrain tile graphics

Couple glitches:

- Acute angle room corners now show as gaps. The hex field of view will not
  reach them, so the wall block that should be drawn there isn't.

- Shooting downstairs makes the gun arc show twice, since the downstairs
  transition shifts things in screen space and the arc occupies both upstairs
  and downstairs locations. Again, this follows from the hybrid logic the engine
  uses.
Generate version string from git
Sketched out a documentation file