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
Merge pull request #393 from TheSriram/bugfix_503

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