Added nginx proxy setting to support Server-Side Events.

Server-Side Events uses chunked encoding.
When using buffering in nginx, it takes too long to update widgets.

See:
https://github.com/Shopify/dashing/pull/86
http://stackoverflow.com/questions/13672743/eventsource-server-sent-events-through-nginx
Removed stages from deploy.rb. stages are automatically discovered.
Added bundler initialization in deploy process.
Setup deploy script using Upstart.