added coordinate_frame reading in from_wcs
bugfix in error reporting
apparently one of my commits restored a previous error
'GLON'->b'GLON' when comparing to wcs
an attempt to parse radesys into coordinateframe.  I think it makes
sense, but I may be mising edge cases
Merge pull request #11 from keflavich/fromwcs_coordframe

added coordinate_frame reading in from_wcs