callbacks for ScalarMappable changed

In the interest of simplifying/unifying the API for callbacks in mpl,
I ported the cm.ScalarMappable callback API to the
cbook.CallbackRegistry functionality which is what we use (on the
trunk) for the toolbar event handling and the Axes xlim/ylim
notification. At a minimum, all callbacks (with one exception that I
am aware of in in mpl are now handled with the
cbook.CallbackRegistry, which is a start. is
passing as are a few interactive tests I tried, but make sure it looks
OK on your end.