First draft of Stripe-based donations.

This PR implements a [Stripe](https://stripe.com)-powered donation form.

Note that if you pull this PR and build it, **it actually works**! It uses a new server called `donate.haskell.org`, which is actually a free Heroku application running my [heroku-stripe-donate](https://github.com/thoughtpolice/heroku-stripe-donate) application to process requests. It uses some personal test-mode keys right now, so donations do nothing (and bogus cards are accepted).

By popular request this form also supports Bitcoin, which Stripe will soon be rolling out to GA.

Currently the form itself is pretty ugly, but I imagine @chrisdone could touch it up and make it look better.