rewrite build server

[cgcgbcbc]

• Write a gin-gonic/gin handler factory to build a gin handler for github hook event.
  Requirements: PayloadHandler need to unmarshal Github webhook event request into struct.
• Implement PullRequestHandler
  Requirements: handler should use channel to send event message to worker
• Implement PushHandler
  Requirements: handler should use channel to send event message to worker
• Implement a git helper
  • clean: clean a git directory by git command, e.g. git stash
  • checkout: checkout to a given branch
  • clone: clone a git repo
  • fetch: fetch some refs
  • pull: pull refs
• implement a worker factory which accept a git helper, an incoming channel, an outgoing channel, and a callback channel, returns a worker function that listen to the channel and run build jobs, send logs to the outgoing channel ,send result to callback channel after jobs done success or failure.
• implement another worker factory which accept a Github token, an incoming channel, returns a worker function that listen to the channel and create Github status.
• the last worker factory accept a path and a channel to listen, then listen to the channel and create files or write contents.