Added Django 1.7 App Config

* Renames the verbose app title to `Python Social Auth` in the Django admin for `Django >=1.7`
![screen shot 2014-10-04 at 10 38 59 am](https://cloud.githubusercontent.com/assets/791000/4515835/4609aca2-4bd5-11e4-9de0-da1092b8860d.png)

* Remains the title `Default` for `Django <1.7`
![screen shot 2014-10-04 at 10 47 20 am](https://cloud.githubusercontent.com/assets/791000/4515841/72128b8e-4bd5-11e4-8606-0f99c56c6a37.png)

Users will still use 
```
INSTALLED_APPS = [
    'social.apps.django_app.default',
    # ...
]
```