Add management command to create state status JSON file

The ``create_status_json`` management command outputs JSON
with information about each state's metadata status and volunteers.

Example usage:

django-admin.py create_status_json > state_status.json

The heavy lifting is done in model methods and a custom manager
to make it easy to migrate if we decide to wire this up to
Tastypie instead of baking static JSON.

I decided that it makes sense to have the management command just create the JSON.
If we want to push it to S3, it makes more sense to delegate that to something like
the AWS Command Line Tools (http://aws.amazon.com/cli/).

Closes #9