Change/allow  hour unit to be 'hr'

It appears that the current default representation for an hour as ``'h'``.  This seems a bit ambiguous, so it would be nice if this could be ``'hr'`` instead.  It might be that the 'h' is required for compliance with one of the formats, though(@mdboom will likely know), in which case it would still be nice if 'hr' could be accepted as a valid string representation (i.e., ``Unit('hr')`` should work).