Add support for Gunicorn/Nginx

This changeset makes Gunicorn the application server for Django across environments. In addition, Upstart handles launching the Gunicorn worker pool and Nginx reverse proxies requests to it.

It is also important to note that when the application is provisioned in the development and test environments, the Gunicorn `--reload` flag is used. This provides automatic Gunicorn worker reloads when code changes occur. I tested application reloading by making source code changes via the Vagrant shared folder and worker PIDs changed as expected.

For more discussion, see: https://github.com/azavea/nyc-trees/pull/71
Sets the static files directory as the web root for this site configuration. This works with `try_files` to determine if a file exists before passing the request through the proxy.