Fix deprecation warnings after Django 1.7 upgrade

After upgrading to Django 1.7 several deprecation warnings are now printed on startup.

<pre>
site-packages/django_mailbox/models.py:76: RemovedInDjango18Warning: `ActiveMailboxManager.get_query_set` method should be renamed `get_queryset`.
  class ActiveMailboxManager(models.Manager):

site-packages/django_mailbox/models.py:353: RemovedInDjango18Warning: `IncomingMessageManager.get_query_set` method should be renamed `get_queryset`.
  class IncomingMessageManager(models.Manager):

site-packages/django_mailbox/models.py:360: RemovedInDjango18Warning: `OutgoingMessageManager.get_query_set` method should be renamed `get_queryset`.
  class OutgoingMessageManager(models.Manager):

site-packages/django_mailbox/models.py:367: RemovedInDjango18Warning: `UnreadMessageManager.get_query_set` method should be renamed `get_queryset`.
  class UnreadMessageManager(models.Manager):
</pre>