Boss, screenshake and stats panel added!

I've added a new boss to the game that spawns at the 5 minute mark. The art and mechanics are placeholder at the moment but they'll improve in the next iterations.

Previously, hitting enemies never felt "right", it was lackluster and missing something. Adding screenshake has added another layer of impact that makes combat feel just that much more visceral. Whack away!

Finally I've added a statistics panel on the end screens so you can see interesting stats from your gameplay session.