Improve serialization

Since [Django 1.5][1], `django.utils.simplejson` is deprecated, use `json` instead. So I changed it in `serializers.py` module (then `DeprecationWarning` message stop). Also other minor optimizations has been done in `django_bootstrap_calendar.serializers.event_serializer` function.

A `TestCase` was created, you can find it in `django_bootstrap_calendar/tests.py` as is the [conventional place][2] to create tests. This test creates an event and checks if serializer return the right json.

The code was commented and doesn't add any additional issue on flake8 runs.

  [1]: https://docs.djangoproject.com/en/1.5/releases/1.5/#system-version-of-simplejson-no-longer-used
  [2]: https://docs.djangoproject.com/en/1.6/intro/tutorial05/#create-a-test-to-expose-the-bug