Game invitations

I think it would be interesting to be able to generate/send invite URLs. I've had several friends I've invited to play on other servers, and the process of registering, finding me/my game, and joining can be a bit involved.

Ideally, I think they should be able to click the link, enter a nickname, and immediately join the game, with an option to join the site after the game. Even if it went through a small registration process before the game, though, this could be useful, I think.

In fact, if you have it check to see if the user's logged in, you could use that URL for email challenges requested in this issue:
#49

[ryanb]

This is a great idea. It is similar to challenging by email (issue #49). Once I find time to work on this some more I hope to add this feature.