Bug fix for cut levels in Info plugin

	modified:   ginga/Bindings.py
	modified:   ginga/FitsImage.py
	modified:   ginga/gtkw/plugins/Info.py
	modified:   ginga/qtw/plugins/Info.py

- now properly handles situation when only one cut level is specified
- added fine and coarse zoom options to bindings
  - ctrl+wheel scroll does a coarse zoom
  - shift+wheel scroll does a fine zoom