webhook

# Check list
* [x] simple webhook
* [x] github
  * https://developer.github.com/v3/activity/events/types/#pushevent
* [x] bitbucket
  * https://confluence.atlassian.com/display/BITBUCKET/POST+hook+management
* [x] gitlab
  * https://github.com/gitlabhq/gitlabhq/blob/master/doc/web_hooks/web_hooks.md
* [x] write reference

# Hook test
```bash
curl  -v -H "Accept: application/json" -H "Content-type: application/json" -X POST -d '{"remote_url": "https://sue445@bitbucket.org/sue445/junitbook-study-rails.git", "branch": "master"}' http://localhost:3000/webhook
```

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