Add amount paid

Amount paid colum is added to ticket purchase to keep the record of money paid by the user.
It is added to physical_ticket#index to show admin, price paid by each user for each ticket.

app/models/ticket_purchase.rb

@@ -52,7 +52,8 @@ class TicketPurchase < ActiveRecord::Base
       purchase = new(ticket_id: ticket.id,
                      conference_id: conference.id,
                      user_id: user.id,
-                     quantity: quantity)
+                     quantity: quantity,
+                     amount_paid: ticket.price)
       purchase.pay(nil) if ticket.price_cents.zero?
     end
     purchase