Add and configure django-allauth package

Dependency on [#3](https://github.com/systers/portal/pull/3).
Configure allauth package. Enable 4 most popular social accounts, the full list can be found [here](http://django-allauth.readthedocs.org/en/latest/#supported-providers). 

Create an empty SysterUser model with a OneToOneField to auth User. Enable creation of SysterUser instance on auth user signup and add a unittest.