Issue #400: add github pushes integration hook

Action needed on the beta server after this push:
- add secret key in config.ini
- create a file web/github_log.txt with the user www-data:www-data

We don't keep the log file in app/logs/ because we want to be able to access the date of the last update from GitHub into the browser
Merge pull request #401 from pascalchevrel/issue400-update_beta_site_with_github_pushes

Issue #400: add github pushes integration hook