First cut at handling different coordinate systems

	modified:   ginga/AstroImage.py
	modified:   ginga/BaseImage.py
	modified:   ginga/Control.py
	modified:   ginga/gtkw/GtkHelp.py
	modified:   ginga/gtkw/plugins/Info.py
	modified:   ginga/gtkw/plugins/Preferences.py
	modified:   ginga/qtw/QtHelp.py
	modified:   ginga/qtw/plugins/Info.py
	modified:   ginga/qtw/plugins/Preferences.py
	modified:   ginga/wcs.py

- Heretofore Ginga just displayed world coordinate info based on FK5
- If AstroPy is installed, Ginga will use it to support different WCS
   coordinate reporting systems: icrs, fk5, fk4 and galactic.
- Preference is currently handled on a per-channel basis (this may change)