Adds two features for MLTP statskeeping.

It adds two features that will be useful for MLTP statskeeping:

* Adds plusminus (score differential) and minutes to the cat-stats extension. (a request from JGibbs)
* Keeping stats after a disconnect (before the following restart) is now way easier: if you clicked on "save as .csv", stats are downloaded when leaving the game page.

It has been tested for about a week now and works great.