Use importlibs

This adds the ability to configure a module for prefixing urls. It accepts a string with dot syntax, in the same manner that other modules are loaded in django settings files. This method uses [importlibs](https://docs.python.org/2.7/library/importlib.html).

Later versions of django have a [helper function for this](https://docs.djangoproject.com/en/dev/ref/utils/#module-django.utils.module_loading). But we are using and older version.