Add a "check" feature

Fixes #9 

This adds a system for voting that will also be helpful for other features like "Reveal" or "Give up". When a user wishes to check an answer (or the entire grid), it's put up for a vote amongst the other participants. When everyone agrees, the last user does the check. A vote has 30s to succeed.

The wave state records a set of (x, y, letter) tuples which have been shown to be incorrect. This lets the UI show either an "X" for an actively wrong answer or a "\" dog-ear for an answer which was previously shown to be wrong.

The voting UI is fairly ugly and there are bugs, but it works!

![screen recording 2014-12-03 at 10 51 am](https://cloud.githubusercontent.com/assets/98301/5283245/6d3f96ea-7ada-11e4-99d3-25fee130da4a.gif)