Fix robots not being able to turn while grabbing tokens
Make tokens behave more realistically

They now slow down and bounce off things.
Give walls friction and restitution

This means that tokens can bounce off them.
Use square graphics for the tokens
Use a square physics entity for tokens
Fix race condition on creating robot physics body
Correct accessors for position/heading of Robots
Use a shared physics lock

This prevents a nasty race condition with creating constraints for
grabbed tokens while running the physics step.
Parameterise the damping of tokens

This allows different arena types to have tokens damped to different
amounts.
Increase the damping for Pirate Plunder

This behaves more like a cardboard token on carpet.
Merge pull request #4 from prophile/physics-improvements

Physics improvements