ArcBall for pyprocessing

Included in the source code repository, and since 16 April 2012 in zip download, I have include a working ArcBall implementation for pyprocessing (that uses Quaternions), for usage see code in particular test_arcball.py, see below for snapshot of running sketch. Mouse dragged rotates the sketch, and if x, y or z keys are held down the axis of rotation is constrained to a single axis.