Simplify package docstring
Simplify class structure

We can know if the decorator is decorating a function or a method.  If
it is decorating a method, then __get__ will be invoked, otherwise it is
decorating a function.
Add list_callbacks() method
Add more examples
Add test for list_callbacks
Add additional test coverage

This covers the case where both handles_exception and takes_target_args
is set
Add tests for class vs instance behavior
Improve test coverage