Add play_match function and command line interface for matches.

Future improvements might include listing of available brains, and a way to store game data easily.
I'd like to see the fetching of the brain function factored out of here. play_match(), like play_game(), shouldn't have to dredge through globals() to find the brain functions.
We'll probably have to do something more sophisticated to import external AIs as well, so if we have the AI-fetching factored out we can put that complexity there.