Fixed a typo.
Added a method to the ChannelMaskPlug which removes channels from a list of channel names until the list contains only channels with a unique channel index.
Added test cases for the above.
Added the ImageStats node along with it's test cases.
Removed an invalid sanity check within GafferImage::ImagePlug.
Made suggested changes.
Made the plug accessors public.
Made minor changes to the hash function.
Added an early-out to the compute method.