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
No need to move into the folder, right?
```
exec("rm {$app_root}/cache/*.cache > /dev/null 2>&1");
```