Removed QUIP specific codes. Disabled auto_levels(), don't think it is necessary. sub_bg() now uses image-modified callback (see ejeschke/ginga#223).
Removed QUIP specific codes. Disabled auto_levels(), don't think it is necessary. sub_bg() now uses image-modified callback (see ejeschke/ginga#223).

spacetelescope/stginga,pllim/stginga