Labels

This will add the labels assigned by GitHub to the issues list.

Currently only labels for new added issues are displayed.

Required updates:
* Database update
* Executing the CLI script to fetch labels for a given project:<br />`tracker.php get project`
* Re-import the issues

Demo [here](http://tracker-joomla54.rhcloud.com/tracker/jtracker-bugs)

There is also an admin management view to list, create and delete labels.