raise ActiveRecord::NotFound when current_order not available in controller

It may not be available when someone manually navigates to /paypal or /paypal/confirm without having a current_order

Fixes #55

Conflicts:
	app/controllers/spree/paypal_controller.rb
Switch .attr() to .prop() for disabling form els

Fixes #54
Fixes #56
Raise ActiveRecord::RecordNotFound if order is not found on cancel page

Fixes #55