Allow choice of rotation; clockwise/anticlockwise

Fixes #105
    
Creates the `rotation` option that allows a setting of `anticlockwise` (or `acw`) 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.

I've tested it successfully using all of the examples founds on index.html and have added the option to one of the existing examples.

**Tested in: Firefox 26, Chrome 32 and on Android 4.4 (Chrome).**