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.

Closes #9
Merge pull request #12 from openelections/9-bake_status_json

Add management command to create state status JSON file