Removed unit conversion for forces, velocities and mass

Forces will stay in Newton, velocities in M/s and mass in kg. Only
positions and sizes will be affected by the pixelsPerMeter ratio.

I did not test all examples yet, but adjusted at least the 'mouse'
example.

This also fixes WheelJoint.setLocalAxisA, which was setting the
localAnchorB instead.

Resolves part of #28.
Merge pull request #41 from bjorn/units

Removed unit conversion for forces, velocities and mass