Add bindGameEvent and shouldStopWalking options

An alternative means to use voxel-walk, tying into voxel-engine's
events. If bindGameEvent is true, will listen on 'tick' events from
the game instance (first parameter passed to constructor). Also
shouldStopWalking will be called, if true will stop the walking
animation, otherwise will start it. Moved code from voxel-hello-world demo.