Added Django 1.7 migrations

I've added initial migration that's required in Django 1.7. I've tried it with most recent RC and it works well. The only problem is that social-auth hadn't used South migrations before and new folder `migrations` is backwards incompatible for users that still use South. AFAIK the only solution should be to create South migrations folder (eg. `south_migrations`) and tell users to use following setting:

```python
SOUTH_MIGRATION_MODULES = {
    'social_auth': 'social_auth.path.to.django_app.south_migrations',
}
```

Or possibly the other way around. Rename this new folder to `django_migrations` and create new `migrations` folder with South migrations. But that seems to be more complicated option.

Feedback is welcome as I'm not 100% sure how to handle this situation.

Related to #270.