Design and create database schema to represent "rule driven" emails ("Reminders")

The workflow for this is described visually in schedule.png on this wiki page:

http://wiki.civicrm.org/confluence/display/CRM/ARMS+Project+Home

The configured email output for each reminder is stored as a "user-driven" message template (with support for tokens).

Ideally the database schema will allow us to also implement things like the below (for future releases):

1. Sending triggered emails based on event start / end date to a set of participants / contacts using CiviMail

2. Sending one or more triggered (reminder) emails similar to the UpdateMembership / UpdatePledge / ParticipantProcessor scripts (i.e. based on membership status and end date, pledge payment status, etc.)