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
Some good detail in there.

I've been using `/bin/false` for a while and was only recently (in the past few years) exposed to `/sbin/nologin`. My decision to go with `/bin/false` has a lot to do with its use by existing UNIX accounts:

```
vagrant@app:~$ cat /etc/passwd | grep false
syslog:x:101:104::/home/syslog:/bin/false
messagebus:x:102:106::/var/run/dbus:/bin/false
landscape:x:103:109::/var/lib/landscape:/bin/false
pollinate:x:105:1::/var/cache/pollinate:/bin/false
statd:x:106:65534::/var/lib/nfs:/bin/false
puppet:x:107:112:Puppet configuration management daemon,,,:/var/lib/puppet:/bin/false
ntp:x:108:113::/home/ntp:/bin/false
colord:x:109:115:colord colour management daemon,,,:/var/lib/colord:/bin/false
nyc-trees:x:999:999::/home/nyc-trees:/bin/false
```

That said, there are several that make use of `/usr/sbin/nologin` too. `¯\_(ツ)_/¯`