Use server sent event

Good bye extra port. #38 
This feature replace websocket with [server sent event](http://www.w3schools.com/html/html5_serversentevents.asp).

`Builder::BuildHandler` is a server responses the build logs by `GET` request as Server sent event stream.
`Builder::BuildHandler` listens on the port `9001`, if you access the `http://localhost:9001/build/master` by `GET` request, the build server starts to build  the `master` container  and puts build log streams.