Add auth middleware

Add middleware to graphite through local_settings.py that allows one to specify a secret key for graphite through a file in a mounted volume.  Modify the Makefile run and debug commands to mount the volume to the correct place and add a README with instructions for adding a secret key to graphite.

If a user chooses not to bother with a secret key, the middleware does not interfere with the container's function.  If it finds a secret key that is not set to the default, the middleware engages and returns 403 Forbidden errors for unauthorized requests.

@shawnrusaw-wf @lyddonb