Gunicorn Changes

Updates to the latest stable Gunicorn version. This is required to use Django 1.5 (see #20). This also drops the use of gevent by default. This was an premature optimization and some initial testing shows it might be much of an optimization at all (see #24). Finally the way the server has been changed to use the built-in wsgi application introduced in Django 1.4 which follows the current recommendation in the docs.