Allows the users to set reminders for themselves

By issuing the commands .in <#.#> <units> <message> or .remind <date>: <message>, a user can set up a reminder after a certain period. Greenman will reply to the user after the period has elapsed and relay the message. Supported <date> formats are [listed here](http://sugarjs.com/date_formats). Supported <units> are strings like "seconds", "days", "m", and "h". Dates are assumed to be UTC.

Reminders can be set far into the future (up to 100 years!), but greenman keeps the next 24 days worth stored in memory. Otherwise, all reminders are stored in a database file to allow greenman to reload the module and still have its reminders. A daily batch job cleans up expired reminders and loads any reminders that fall into the 24 day range.