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)
I don't think the full response content should be dumped to screen.  If it is necessary to print something, use `print()` and `from __future__ import print_function` at the top (for py3 compatibility).  This is the reason for the test failure.