Adds debug.print_exc()

traceback.print_exc() prints the frames from the catching point to the
raising point. This new function prints frames above the catching point
as well.
print_exc() refactoring: adds format_exc()
Makes unexpected_exception() use print_exception()
Changes calls to traceback to calls to debug

When a full traceback is required, use debug.print_exc() and
debug.format_exc() instead of traceback functions.
Merge pull request #28 from VisTrails/show-full-traceback

This will be useful. I often have to improve tracebacks to get useful information!