Add a simulated physics world to the Arena

This will be used for simulation of Robot behaviour.
Mark the simulator thread as daemonic

This means that Control-C properly kills the process.
Use Box2D physics for the Robot

This leads to some ballet at the moment rather than the expected behaviour, patches for that incoming.
Resist lateral velocity on robots

This helps with the ballet slightly.
Account for wheel friction
Expand to an 8mx8m arena
Add the arena walls
Removing the pre-ready position setter warnings
Make the tokens physics objects
Add grabbing capabilities to the robot
Merge branch 'new-physics' of github.com:prophile/sr-turtle into prophile-new-physics

Conflicts:
	sr/arenas/arena.py