github integration(experimental)

#63 a little integration with GitHub.
- `./bin/github_bot` runs as service, watches new pull requests on preview target repository, and puts comment preview url on the pull request.
- Added the document `docs/github-integration.md` how to use `./bin/github_bot` to integrate with github.

Other changes:
- `supervisord` didn't work normally, for example, `supervisorctl restart build_server` command didn't return forever, then update supervisord version 2 to 3, and fix config.