Allow computer-controller players

[stevegrossi]

Woo!

I've got a long way to go, though (in future PRs):

• [ ] Make AI a lot less naive
• [ ] Different kinds of AI? Aggressive, defensive, etc. (maybe @behaviour Sengoku.AI)
• [ ] Some way to let human players add AI players before game start
• [ ] Refactor ai.ex so it's clearer how AIs think (and easier to tell how to change them)
• [ ] Some way to test AIs against each other without watching in the browser. Maybe a mix task that runs several GameServers concurrently? I'd like to make it easy for contributors (myself even) to add new AI modules and see if they're any better at the game than existing ones

[stevegrossi]

I look forward to improving on this, but it's a nice start for now!