Enhancements to Ginga FITS viewer

	modified:   AstroImage.py
	modified:   PythonImage.py
	modified:   gtkw/plugins/Pick.py
	modified:   iqcalc.py
	modified:   qtw/plugins/Pick.py

- Added "background" to Pick plug in
  - this is a more standard definition of background level based on median
- Updated PythonImage to use QImage if it is available
  - this is handy for Mac or Windows users who don't readily have PIL available