Django 1.8 support

[wengole]

Currently getting AttributeError: 'Options' object has no attribute 'module_name'

Since the Django 1.8 the model _meta now has a defined API. It appears this no longer explicitly defines a module_name attribute.

Traceback (most recent call last):
  File "/home/benc/PycharmProjects/wizfs/src/snapshots/tests/test_models.py", line 48, in test_str_method
    modified=timezone.now()
  File "/home/benc/buildout-cache/eggs/Django-1.8-py2.7.egg/django/db/models/manager.py", line 127, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/home/benc/buildout-cache/eggs/Django-1.8-py2.7.egg/django/db/models/query.py", line 348, in create
    obj.save(force_insert=True, using=self.db)
  File "/home/benc/PycharmProjects/wizfs/src/snapshots/models.py", line 104, in save
    super(File, self).save(**kwargs)
  File "/home/benc/buildout-cache/eggs/Django-1.8-py2.7.egg/django/db/models/base.py", line 710, in save
    force_update=force_update, update_fields=update_fields)
  File "/home/benc/buildout-cache/eggs/Django-1.8-py2.7.egg/django/db/models/base.py", line 747, in save_base
    update_fields=update_fields, raw=raw, using=using)
  File "/home/benc/buildout-cache/eggs/Django-1.8-py2.7.egg/django/dispatch/dispatcher.py", line 201, in send
    response = receiver(signal=self, sender=sender, **named)
  File "/home/benc/buildout-cache/eggs/django_haystack-2.4.0.dev0-py2.7.egg/haystack/signals.py", line 52, in handle_save
    index.update_object(instance, using=using)
  File "/home/benc/buildout-cache/eggs/django_haystack-2.4.0.dev0-py2.7.egg/haystack/indexes.py", line 282, in update_object
    backend.update(self, [instance])
  File "/home/benc/buildout-cache/eggs/xapian_haystack-2.0.0-py2.7.egg/xapian_backend.py", line 470, in update
    (obj._meta.app_label, obj._meta.module_name, obj.pk, data),

[jorgecarleitao]

Thanks for adding it here. I think we can use the solution proposed here.