Stronghold

Instead of #19 use `django-stronghold` (https://github.com/mgrouchy/django-stronghold).

I tested this with the current plinth master, python3, django1.7-2 and django-stronghold==0.2.6, installed via pip3.

`setup.py` and `INSTALL` should be updated before merging this pull request, @SunilMohanAdapa please tell me when the packages (or names) are ready.