For Django 1.5 compat, remove markdown support.

`django.contrib.markup` is now on an accelerated deprecation schedule [according to Django 1.5 release notes](https://docs.djangoproject.com/en/dev/releases/1.5/#django-contrib-markup). I believe this to be the reason why I am getting the following error when trying to use pyjade with Django 1.5rc1: `Error importing template source loader pyjade.ext.django.Loader: "No module named markup.templatetags.markup"`. 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 Django 1.5 compatibility.