Bug fixes and upgrades to Ginga

	modified:   AstroImage.py
	modified:   AutoCuts.py
	modified:   BaseImage.py
	modified:   FitsImage.py
	new file:   LayerImage.py
	modified:   Mixins.py
	modified:   PythonImage.py
	modified:   RGBMap.py
	modified:   cmap.py
	modified:   ginga.py
	modified:   gtkw/GtkHelp.py
	modified:   gtkw/plugins/MultiDim.py

- added "rampclip" color map
- added LayerImage mixin by user request
  - allows layered images (e.g. transparency for colored bit plane masks, etc.)
  - "Layers" plugin coming soon
- bug fix for a broken splash screen