Add host to default_url_options in app routes

This allows any object to generate full URLs based on the application
routes by including Rails.application.routes.url_helpers.
Add support for AWS payments (Amazon Simple Pay)

This will be the new method of making contributions for send-money donors.
Enable live AWS payments in prod

But also include a way to override that to use the sandbox in prod for
integration testing.
Add a ContributionsController using AWS payments

* /contributions/new replaces /donations/pay
* send-money reminders don't need explanations about "recording" donations
Fix AWS Payments notification URL

The "IPN" URL that gets a POST should be the create URL, not new
Don't require login for contribution create

Since this is hit by AWS Payments, it doesn't have login cookies.
Upgrade to Rails 3.1.11, JSON 1.7.7
Verify AWS signature before creating Contribution

Also refactor some of the AWS integration code.
Don't specify the CA bundle file

This isn't working on Heroku.
Add transaction ID to contributions

Migration only.
Actually create Contribution from Amazon payment
Don't show thankyou for a failed payment

Redirect to the “checkout” page instead.
Remove obsolete line from test form
Don't create contributions from fake transactions