Implement Mailbox-style Snoozes

Snoozes rock. The concept is this: You want to get to Inbox 0, but sometimes there's a message you just can't delete yet. Snoozes allow you to stash a message away for Later. You can deal with that message tonight or tomorrow or next week.

We should find a way to implement snoozes that doesn't require running a server. Mailbox seems to use labels for this. When a snooze is applied, we move it to a label (out of inbox). The client should check snoozed messages and move them back into your inbox at appropriate times.