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
Ok.  The static directory isn't the right directory to serve static assets from though.  `static/` is where I intend gulp to put them into, once we wire it in place, but they'll get **collected** into `assets/` (though [a recent PR of mine changes](https://github.com/maurizi/nyc-trees/commit/412d892949beeb92a0bc317329e61dd62682de42) this to the more obviously named `collected_static/`)