Added `Upgrade` and `Connection` headers to the template to allow nginx to [upgrade a request](http://nginx.com/blog/websocket-nginx/).
```
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
```
These headers are only added if a `WEBSOCKETS` environment variable is defined.
Should address #6.
The port selection now works as follows:
* If there is only 1 port exposed by the container, that port is used.
* If there is a VIRTUAL_PORT env variable defined, that port is used.
* Otherwise, default to port 80, if exposed.
