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

Allow users to query a list of coordinates for SDSS query