Add STOP_ON_WARNING option to top level Makefile, INSTALL
Add documentation on Python binding for IBA::cut
oiiotool --over and --zover should set output display window to union, not fg

I hadn't noticed a problem before because fg and bg are usually the same
size. But if they differ, it seems more likely that it's a small element
being put overtop a fixed background, so setting the output's display
("full") window to the fg is not what you want.