rake task to send reminders

Added a workshop:send_remiders rake task that will be responsible for
sending email reminders to everyone enrolled in a section that is
starting tomorrow.

Added a :starting_tomorrow scope to the Part model that will return all
parts that start tomorrow.
rake task and mailers

Added a rake task "workshop:send_reminders" and a
UserMailer#send_reminder action to email reminders to users enrolled in
courses starting 'tomorrow'.
Send reminder 3 days in advance

So I just looked at the Trello card, and the request was to send the
reminder 2-3 days in advance.  So that's what we'll do :)
Merge pull request #20 from cit-jmu/reminders

Reminder emails