Added a MessageMixin.

As mentionned to Kenneth on twitter, I originally considered adding this to Django itself but I think this works nicer as a third party app.

All tests seem to pass on my machine, except those involving Python 2.6 because I don't have it installed.

Support for Django 1.4 is a bit tricky because the `request` attribute is only set on the view during `View.dispatch()` which means it's not available very early.
I "solved" it with a big warning box in the documentation and skipping a test that was checking just for that.