Trigger job via GitHub comments

[alexAubin]

Tested here https://github.com/YunoHost-Apps/helloworld_ynh/pull/1 and http://ci-apps-dev-v2beta1.nohost.me/

Basically you say !testme in a PR comment and that'll trigger a test on the CI. This will only be accepted if you're flagged as member/owner of the repo/organization in Github. The big plus is that this should require very loose maintenance / configuration in every day life :

• the webhook configuration is global to Yunohost-Apps
• no need anymore to ask people an SSH key to get access to the dev CI
• no need anymore to run a weird bash script with rsync magic then wait 5 minutes for the cron job to pickup your job (though now you kinda must create a branch+PR to be able to trigger the job though)

Still planning to build on this such that the CI uses the yunobot account to answer with a comment with a cool jenkins-like badge displaying the job status + link to the created job

Also probably more polishing to better handle exceptions :s

[alexAubin]

Yolomerging because i'm deploying this on a production dev CI ... but we can keep iterating if there are comments