Can now interpolate with HSV!

Added function: utils.interpolate_hue for interpolating hue degrees on
a circle (as in, it wraps around)

Fixed the sorting order in Horizon.sort_hooks (it was wrong)

Corrected my own stupidity in utils.convert_to_rgba

utils.rgba_to_css now correctly outputs "rgba(" rather than "rgb("

utils.hsva_to_css handles negative hues and hues over 360; plus, fixed
a nasty syntax/logic error

Horizon is now set to HSV mode by default -- also changed all
occurrences of HSL to HSV