Use glicko instead of Elo

Glicko is better than Elo because it gives more accurate rankings and has the concept of uncertainty in your ranking. It can do clever things:
- It isn't zero-sum. Your ranking changes less when it knows more about you, and changes very fast when you're new
- It takes account of dates matches were played (this would require a database fix of course), so if you haven't played for a while, it becomes more uncertain about you and changes your ranking more.
- Even if you've played lots, then if they're all against the same person it's still quite unsure about you.

You can get a standard glicko javascript implementation online. glicko2.js in the current codebase is what to use - https://github.com/mmai/glicko2js.

This is definitely a WIBNI as Elo is perfectly acceptable, but if we're making things totally awesome then why not do this?