Added offset support for BoxShapes

[SleepProgger]

This PR mainly adds support for offset to BoxShapes by extending them from Poly instead of Shape. This is basically what is happening in chipmunk too. cpBoxShapeNew is just a helper function calling cpPolyShapeNew and ignoring the offset.

Additional changes in this PR:

• Added an offset property with a getter to Poly.
• Removed the unused radius parameter fromPoly.create_box and use the offset argument in the Poly constructor call.
• Added a bit of the pymunk docstring for the Shapes friction property. Sorry for this unrelated change, but imho that is a really good piece of information to have there.

The only two cases i can thing of which might break due to this changes are:

• If someone used the (unused) radius argument of Poly.create_box
• Code like this:

  if isinstance(shape, Poly): # Would now match for BoxShapes, while it wouldn't before
  do_something()
  elif isinstance(shape, BoxShape)
  do_something()