refactor _is_coordinate / _is_name tests to common location, and avoid
issue astropy#1727
isinstance tests fail; use other ways to test... (I prefered
isinstance..., but coordinates.Galactic !=
coordinates.GalacticCoordinates)
more isinstance -> equality tests
py3 compat?
Merge pull request #248 from keflavich/fix_coords_nov2013

Bug fixes for recent astropy coordinates changes