Expand SDSS query

Expanded the SDSS by adding photoobj_fields and specobj_fields to
query_region_async and _args_to_payload parameters so that the user can
retrieve any field from the PhotoObjAll object (see
http://cas.sdss.org/dr4/en/help/browser/description.asp?n=PhotoObjAll&t=
U)
Perhaps this should be a [RemoteServiceError](https://github.com/astropy/astroquery/blob/master/astroquery/exceptions.py#L30)?