fix(errors): HTTPServiceUnavailable error's retry_after can be datetime

The class should now be converting retry_after of type 'datetime' to HTTP
Date through falcon's util.dt_to_http(retry_after). This patch also
updates the corresponding docstrings.

Closes Issue: https://github.com/racker/falcon/issues/392
comment updated to use bound methods in the example.
Merge pull request #393 from TheSriram/bugfix_503

fix(errors): HTTPServiceUnavailable error's retry_after can be `datetime...
Merge pull request #394 from eerwitt/master

Middleware object example using bound methods
test(pep8): Increase max mccabe score from 12 to 15

Ocassionally we need to increase complexity in order to deal with
PY2 vs. PY3, or to optimize hot code paths.
chore(AUTHORS): Added eerwitt. Thanks for contributing!