Modified to use astropy module if it is installed

	modified:   ginga/AstroImage.py
	modified:   ginga/gtkw/plugins/FBrowser.py
	modified:   ginga/gtkw/plugins/MultiDim.py
	modified:   ginga/qtw/plugins/FBrowser.py
	modified:   ginga/qtw/plugins/MultiDim.py
	modified:   ginga/wcs.py
	modified:   scripts/example1_gtk.py
	modified:   scripts/example1_qt.py
	modified:   scripts/example2_gtk.py
	modified:   scripts/example2_qt.py

- otherwise falls back to individual pyfits and pywcs modules