email previews + Remindercategories

OK, ready to merge this stuff.

These ended up getting tangled up a bit more than I would have liked but are pretty straightforward modifications

1) Email previews.  Create a codepath controlled by Reminder::send_all(render_only=True) which simply renders emails that would have been sent out had SEND_EMAILS been True.

I really wanted a way to see what emails uber is trying to send before flipping SEND_EMAILS back on, and without having to send a bunch of test emails.

You can see this at http://staging.uber.magfest.org/magstock/emails/preview_tosend

2) Reminder categories - add category support to Reminder class so that we can turn on and off different email categories based on the config file

Useful because, for example, Magstock doesnt need any of the dealer, staffing, etc emails.

By default all the un-categorized Reminders have category of 'uncategorized' (most of them are still this)

uber will still send allow all Reminders if the allowed categories include 'all', which is the default in the config file.

fixes #236