Follow-up potentially sent to non-followers

in https://github.com/danirus/django-comments-xtd/blob/master/django_comments_xtd/views.py#L144, the query selecting followers is not filtering by content-type.

Consider the following scenario:
  1. user Charlie is subscribed to model Diary, pk=1
  2. user Bob is subscribed to model Story, pk=1
  3. user Alice comments on Story, pk=1
  4. both Charlie and Bob receives a followup email

The fix should be pretty and I wrote a test for it, but it seems to hang on when calling `mail_sent_queue.get(block=True)` and I can't figure out why. Testing on Python 2.7.6 and happens both on Django 1.5 and 1.6.