Fixes in requirements.txt and default.py

I just started a new project and used tangent-kickstart and tangent-django-boilerplate. I realised that two things weren't working correctly:

1. Running ``runserver_plus`` provided by the ``django_extensions`` package does not work as it cannot find ``Werkzeug``. I added it to the ``requirements.txt`` file.
2. When running tests using django's test runner, they brake due to ``django_extensions`` which tries to ``append`` its test cases to ``INSTALLED_APPS`` which is defined as tuple in ``default.py``. I changed that to be a python list.

I thought I'd share these changes so that someone else doesn't run into the same issue :) Hope the fixes are OK. 

Cheers,
Seb