Add explicit app_label

[rgaiacs]

This pull request will solve #11.

Test instructions

To test the pull request with my broken app I had to install https://github.com/python-social-auth/social-app-django/ and https://github.com/python-social-auth/social-core.

$ git clone https://github.com/python-social-auth/social-core.git
$ cd social-core
$ python setup.py install
$ cd ..
$ git clone https://github.com/python-social-auth/social-app-django.git
$ cd social-app-django
$ python setup.py install

Extra changes

I had to replace

'social.apps.django_app.context_processors.backends',

with

'social_django.context_processors.backends',

and

url('', include('social.apps.django_app.urls', namespace='social')),

with

url('', include('social_django.urls', namespace='social')),

on my broken app.

[omab]

Thanks!

[ticosax]

Are you sure this should have been merged ? the error was due to a misconfiguration.
app_label is already "social_django". We don't need to hardcode it.

[omab]

It's not required but doesn't make any harm also.