Prevent unnecessary digest cycles.

Without this change, the new handler throttling added by @oblador in #53 will cause a digest cycle to be run with every scroll callback, negating any performance advantage.  The handler doesn't appear to need to be run inside a digest cycle, since the only Angular-related code is the broadcasting of the becameInactive and becameActive events, but the sample code in the README.md makes clear that listeners need to force their own digest via $scope.$apply().

I also removed an unnecessary digest in scrollspy.js, but that one doesn't have any performance implications.

AFAICT these changes don't break anything (my app works as before) but I was unable to run regression tests so caveat emptor.