Remove soon-to-be-deprecated markdown filter from Django extension

This commit removes the import of and use of the markdown filter from `django.contrib.markdown`. This is to ensure full Django 1.5-1.6 compatibility, and addresses half of issue #62.

Of course markdown support could be reimplemented in a commit to follow this one, but it should be handled with a 3rd party library to ensure full Django 1.5-6 compatibility.