Change default to SDSS III site (original CasJobs is closing down)

If you go to http://skyserver.sdss.org/casjobs/, you can see the following comment:

> this site will be retired On July 31, 2014 to be replaced by the SDSS-III CasJobs site 

with that in mind, this PR just changes the default `base_url from the original SDSS I/II CasJobs to the SDSS III CasJobs (as well as mentioning a bit more in the docstring about it)