added utility function urljoin_keep_path
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)
Display error to user

Fixed bug where an error message from the server caused the code to
throw a ValueError exception without displaying the error message from
the server
Updated print function for py3
Changed exception to RemoteServiceError
Removed encode('ascii') for py3 compatibility
Forgot one of the encode('ascii') lines
Attempting to fix py3 encoding error
Merge pull request #375 from fred3m/master

Expand SDSS query
Allow users to query a list of coordinates

Due to restrictions by the SDSS server it can be useful to query a list
of coordinates in a single query. The result can then be matched with
the original list by using:

dr = coords.Angle(radius).to('degree').value
obj_coords = coords.SkyCoord(result['ra'], result['dec'], frame='icrs',
unit='deg')
matches, d2d, d3d = obj_coords.match_to_catalog_sky(pos)
result = result.group_by(matches)

where the group keys are the indices to the coordinates sent to the
query.

A future modification of astroquery can do the above procedure
automatically.
corrected missing brackets, units, and parenthesis
Changed SkyCoord to commons.CoordClasses
use six library for Py3 compatibility
adjusted documentation for utils.url_helpers.urljoin_keep_path to latest changes
break lines after 74 chars in docstring
Merge branch 'master' of github.com:astropy/astroquery into urlutil
Merge pull request #373 from derdon/urlutil

added utility function urljoin_keep_path
Changed check for scalar
Merge pull request #376 from fred3m/SDSS-batch-query

Allow users to query a list of coordinates for SDSS query
irsa_dust.rst clarifications and addition of a link