Shortcut to assign stop_max_attempt_number

[davidyen1124]

Make some changes so we can use @retry(3) or @retry(stop_max_attempt_number=3).

[eliasdorneles]

This is a neat idea, interesting implementation hack! :)
So, I'm not a maintainer of retrying, but it would be nice have tests for this.
Good test coverage is pretty important for retrying, since the library is mainly about reliability and is also used in the core of many projects.