Improve the test coverage on models

I can't get the test runner to include the function definition lines,
as per this issue - https://github.com/kmike/django-coverage/pull/10
so we'll never get to 100%, but am nearly there in practice.
Add templated render method to callback events

The major use case (for me) for this app is to convert the JSON returned
from Trello into a message and then send it somewhere else. The new
render method uses the django templating to render the JSON payload as
the template context.

The templates are in /templates/trello_webhooks/<event_type>.html, and
can be overridden by other apps, so that users have full control over
the output. The default templates are very functional, so should be
treated as samples only.
Add travis CI integration