Update event_payload field to JSONField

Also updates the requirements to include django-jsonfield
Add some sample test data from the Trello API

I've added in some static JSON files taken directly from the Trello API
using the public test board https://trello.com/b/TAAnwdP9/django-trello-webhooks-test-board

* commentCard
* createCard
* createList
* updateCard (fired when moving between lists)
* updateComment
Minor update to logging in view function

Update from DEBUG to INFO for the callbacks
Add convenience methods for accessing callback JSON

The CallbackEvent now includes:

* action_data - the ['action']['data'] node
* action_member_fullname - ['action']['memberCreator']['fullName'] node
Admin site improvements on CallbackEventInline

Added the name of the person responsible and the 'data' portion of JSON
Add properties to CallbackEvent to parse payload JSON

Added board, list and card properties, which are JSON extracted from
the underlying event_payload JSON (from the action node). They may or
may not exist, depending on the event_type, but are handy for display
purposes.
Improve admin display (add board, list and card names)
Add some tests!

OK, so we're only up to 12% coverage of one module (models), but it's a
start. We have a working tox.ini file as well, so you can just run it
through tox.