fix deprecation/remove of django.contrib.comments in Django 1.6

In Django 1.6, Django’s comment framework has been deprecated and is no longer supported. 
See https://docs.djangoproject.com/en/1.6/ref/contrib/comments/ for details.

In order to make django-fake work with django 1.6+, i just add verification before import ``Comment``
by checking if ``django-contrib-comments`` is into ``settings.INSTALLED_APPS``.