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).
Honestly I'm not sure whether this feature test is useful above and beyond the notifier test.  Probably not needed.