Add front page for waiting while building Docker image and running it #6

I changed code to add front page while building Docker image. #6 

1. Divide build module to `builder/`
2. Showing font page while building Docker image and run it
  * Return a html to show build log. It communicates with `build_server.rb` in proxy server by Websocket
  * Add `build_server.rb` to handle this. It gets request from front page then launches some build tasks. Logs are sent from `build_server.rb` through Websocket. 

@ainoya Could you check it out? Thanks :beer:
It's may be fail to find `gem path`.
Please use `
sudo sh -c 'find /opt/ruby-2.1.2/bin/* | xargs -I{} ln -s {} /usr/local/bin'`` instead of  `ln -s /opt/ruby-2.1.2/bin/ruby /usr/local/bin/ruby` on line https://github.com/mookjp/pool/blob/add-build-front/Vagrantfile#L39 .