Moved RGBChannelTransform to RotateHue and removed RGBImage.

Changed RGBChannelTransform's name to RotateHue to reflect what it
actually does.

random_hue_jitter RandomDistribution to just "jitter", a Number,
because jittering hue is all this class does, and because it doesn't
necessarily need to be random.

Removed apply_hue_jitter Boolean; now achievable by doing jitter=0.

Docstrings indicate that the class does not really require RGB.