Use astropy's caching

This is to address Issues #23, #19.  It replaces the progressbar code with astropy's progressbar.

At the moment, astropy's `get_readable_fileobj` doesn't have a non-verbose option, which would be nice (but this may still be mergeable without that feature).

Not all of the querying has been replaced with astropy's routines, as some were wrapped up in multiprocessing, making it somewhat non-obvious how to proceed.  Will astropy have a parallel downloader at some point?  (see `ukidss.py` for parallel download examples)

Also, probably needs to be rebased...