Added comment about execute

Executing a payment does *not* update the payment object with shipping address. You have to do Payment.find(...) again to retrieve that information.

Example:

```ruby
payment = Payment.find("PAY-57363176S1057143SKE2HO3A")

if payment.execute( :payer_id => "DUFRQ8GWYMJXC" )
  payment = Payment.find("PAY-57363176S1057143SKE2HO3A")
  # now shipping address is available
end
```