Minor modifications to API of read_nbody to improve behavior of test suite.

Now, the ask_permission boolean governs whether the user is prompted for permission
to download the halo/particle catalog.
Beginning to revamp read_nbody. Currently committed version is likely broken.
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.
Narrowing in on completing the catalog manager and revamping read_nbody.
Nearly done with processed_snapshot object in read_nbody
Improvements to UI of the catalog manager
Added method to load catalogs into memory from explicit filename input
Convenience attributes added to Catalog_Manager object
Minor modifications to API of load_catalog method of Catalog_Manager
processed_snapshot object now correctly talks to catalog_manager
Integrated optional download behavior into processed_snapshot object
Modified default download behavior of processed_catalog object
Added method to attach simulation characteristics to processed_snapshot object
HOD_mock object now gets simulation data via processed_snapshot
Modified test_Zheng07_mock to use new catalog_manager behavior
Temporarily commented out mock-making test