Added gwpy.time.gps_types tuple

This PR adds the gwpy.time.gps_types tuple, similar to six.string_types, which contains all available, known implementations of LIGOTimeGPS, e.g.:

>>> from gwpy.time import gps_types
>>> print(gps_types)
(<type 'lal.LIGOTimeGPS'>, <class 'ligotimegps.LIGOTimeGPS'>, <class 'glue.lal.LIGOTimeGPS'>)

This is mainly useful for isinstance checks.

This includes a unit test that the included gps types are all API compliant.