Planting reminder

This code sends out a reminder email to members every 2 weeks, asking them to update their plantings/harvests.

PT story: https://www.pivotaltracker.com/story/show/55884726

The important stuff is around the notifier and app/views/notifier/planting_reminder.html.haml as well as the rake task that actually sends the emails.

There is a little bit of superfluous work to do with "interesting" plantings which is sort of part of the next stage (https://www.pivotaltracker.com/story/show/75317358) which I could remove if anyone objected to it being in here (since it's not really used for anything yet).
Note to self: I should be using site_name throughout this file, not calling ENV all the time! *headdesk*