Refactored FitsImageGtk to subclass FitsImageCairo

	modified:   ginga/AstroImage.py
	modified:   ginga/cairow/FitsImageCairo.py
	modified:   ginga/gtkw/FitsImageGtk.py
	modified:   ginga/wcs.py

- also includes a fix for a bug where Ginga was failing to load images
     with no WCS info