Hacked the astropy utils code to create some configuration utilities.

There are now a few global directories where halotools data is stored,
all of which are subdirectories of the astropy cache directory. So the
code is now becoming more closely intertwined with astropy.
Deleted unnecessary methods from Config object
Continuing reconfiguration of catalog management system
Added method to identify best matching catalog in cache
Still working on processed_snapshot object in read_nbody. Hardest parts are now done.

Should create methods to read from (and create, when necessary) 'halos' and 'particles'
subdirectories of the halotools subdirectory. This is where I should store my
pre-processed data.

Also still need to finish writing the 'halos' and 'particles' methods
of the processed_snapshot object. Once that's done, read_nbody will pretty much be in business.