Add TimeFormatMeta and TimeDeltaFormatMeta

 These 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.