Add option 'allow guest orders' to enterprise

An enterprise manager can choose to allow guest orders (default) or
require a valid login for checkout.

db/migrate/20160921060442_add_allow_guest_orders_to_enterprise.rb

@@ -0,0 +1,5 @@
+class AddAllowGuestOrdersToEnterprise < ActiveRecord::Migration
+  def change
+    add_column :enterprises, :allow_guest_orders, :boolean, default: true, null: false
+  end
+end