Assumptions: NFL pick 'em pool. One wrong pick and you're out. Play until
there's one person left, the winner.

It should work something like this:

 * You arrive at the site, want to start a pool for you and your friends.
   Sign in, start a Pool, set the rules, invite players. You are the manager
   of that pool.

 * When a player joins a pool, they're creating an Entry into that pool.  That
   entry will last until they pick incorrectly.  The Entry will be marked paid
   or unpaid by the manager.

 * Making picks: We show the players every game left on the schedule, broken
   down into Slates (ie, a slate of NFL games is one week's games).  The
   player can pick as far into the future as they want.

 * When a user picks wrong, they will be notified and given a chance to create
   another Entry into the Pool (depending on the pool's rules). They'll still
   be able to watch what happens in the pool, regardless.

 * If, god forbid, the Pool is still active at the end of the season, the
   consequences are undefined.  It'll be up to the Manager to sort it out
   offline, by asking for score guesses for the final game or carrying the
   Pool into the next season or something.


Per-pool options:

 * Picking straight up or gainst the spread

 * Can you buy back in?

 * Deadline for making picks? A hard deadline (e.g. Thursdays at noon) or can
   you make a pick as long as there are any games left that haven't started
   (ie, you can pick from any of the current slate of games that have not yet
   kicked off)?