Paypal Integration

Notes from: https://groups.google.com/d/msg/rubyparley/7grAQjnjwe0/A6PcFITW9U4J
Also see: http://railscasts.com/episodes/146-paypal-express-checkout

Use ActiveMerchant::Billing. When you're ready to have someone checkout, send them to a rails controller action Paypal#checkout. This will:

0. "gateway" is a ActiveMerchant::Billing::PaypalExpressGateway.new
1. send a message to gateway.setup_purchase.
2. #setup_purchase has a method redirect_url_for that you'll use to redirect the user to Paypal. 
3. Paypal will redirect the user back to Paypal#complete
4. on the complete page, you show the user the order form again with a finalize button
5. Paypal#finalize will call gateway.purchase with the token -- so you could finally take their money with

      @purchase = gateway.purchase(session[:total_in_cents],
                                   :ip       => request.remote_ip,
                                   :payer_id => session[:payer_id],
                                   :token    => session[:token]
                                  )
      unless @purchase.success?
        raise ActiveRecord::Rollback
      end