Added GafferImage::RemoveChannels.

* Added the GafferImage::RemoveChannels node to allow channels to be removed from the image stream by specifying a selection to keep or remove.
* Added test cases for the above.

Note: This pull request is dependant on the cortex pull request #59.
Added an Enum for the two RemoveChannels modes and updated the test cases for clarity.
Simplified the hashing of the channel names.
Removed unnecessary flags on the ChannelMask knob.
Moved the RemoveChannelsMode enum into the RemoveChannels scope.