Phases and groups and rounds, oh my!

Fixes #2, #14, #15
I'm not exactly enamored of "Group", but I rejected "Pool" because I think people might want to use this object differently. For instance, if we hypothetically started keeping stats for consolation matches at the HSNCT, we'd probably have one `Group` for the real playoffs and one `Group` for each consolation mini-tournament, or similar. Would you buy that?