move stripe API call to model, schema changes

db/migrate/20160606040848_create_payments.rb

@@ -1,9 +1,9 @@
 class CreatePayments < ActiveRecord::Migration
   def change
     create_table :payments do |t|
-      t.string :last4, null: false
-      t.integer :amount, null: false
-      t.string :authorization_code, null: false
+      t.string :last4
+      t.integer :amount
+      t.string :authorization_code
       t.integer :status, default: 0, null: false
       t.integer :user_id, null: false
       t.integer :conference_id, null: false