Add TimeFormatMeta and TimeDeltaFormatMeta to automatically add TimeFormat classes to the TIME_FORMAT and TIME_DELTA_FORMAT dicts at class creation time instead of after the fact.  This should also reduce the effort required to define new formats external to Astropy.  This may understandably look overengineered, but it does have the advantage of fixing the annoyance mentioned here: https://github.com/astropy/astropy/pull/3095#issuecomment-65074252 by not doing any module-level hackery.
Merge pull request #3169 from embray/time/time-format-registry

Add TimeFormatMeta and TimeDeltaFormatMeta