Documented Stalkerbot and Playerbot

Also converted remaining classes to new-style classes.
Fixed bug with Navbot not terminating old path after setting new destination

PS: Now Stalkerbot is even scarier
Shrunk realbot, decoupled realbot-virtualbot instantiation

Lots of small things in this commit. First:
- A realbot is now 16x16 pixels. This makes the visual effect of the sprite
  look less disproportionate, and lets the bot feel smoother to control.
- Instantiation of realbot and virtualbot is partially decoupled. The realbot
  can be instantiated without a virtualbot, but the virtualbot at the minimum
  still needs the position/size of the realbot at initialization time.
- As a consequence of the above, the arena_data dict passed onto to virtualbot
  at init is more useful. It contains the positions of walls, the arena itself,
  and the realbot. Virtualbot.__init__ helpfully saves these into instance
  attributes so subclass don't have to explicitly do so.
- Removed stupid leading underscores in front of "private" methods.
Converted Playerbot to a subclass of Queuebot