Allow choice of direction; clockwise/anticlockwise

Fixes #105

Creates the `direction` option that allows a setting of `anticlockwise`
to reverse the direction of progression used by the knob. It fully
supports the clicking/touching and `displayPrevious` functionality.

To facilitate extending this behavior to custom draw functions, arc
calculations have been factored out of `draw` into an `arc` function,
which returns the start and end angles, and a boolean denoting the
direction of the stroke.