Statuses

This will add the "merge status" from GitHub (e.g. currently the result of the Travis run)

In the future it should be possible to add our own status and push it to GitHub (This is, when the "bot account" is ready (CC @allenzhao :wink:))
May be it is better to move `getMergeStateClass()` and `translateMergeState()` [here](https://github.com/joomla/jissues/blob/master/www/jtracker/core/js/jtracker.js) ?