Replaced uwsgi with gunicorn (reason: downloads were not reliable with uwsgi)

gunicorn is built by buildout and has one external dependency: libevent
there is a configuration file, gunicorn_settings.py.
Command to run gunicorn: bin/gunicorn -c gunicorn_settings.py wsgi:application
updated documentation with new dependencies
added ext dependency to travis.yml
updated gunicorn documentation and foreman scripts. scripts still require testing
another change to .foreman file was needed
improved install docs for gevent libs
Merge pull request #292 from grischa/gunicorn

Gunicorn provides reliable downloads, uwsgi doesn't. This replaces uwsgi