Attribute issues_closed to closing user, rather than author

[chillu]

Github doesn't publish this info as part of the issues API
(see https://api.github.com/repositories/1318892/issues?since=2014-02-13+21%3A59%3A27+UTC&state=closed&page=1). We'll need to look at the events API instead, which only tracks the last 300 events, so will require some form of database to collect those and evaluate 60 days of events (which can easily be 1000+, given this includes repo pushes). This will make the leaderboard a bit more balanced towards regular issue warriors like @simonwelsh :)