Introduction of smpp 3.4 spec compliant err code on delivery receipts.

I've added the #rawErrorCode on DeliveryReceipt as a String.  If the #setRawErrorCode can parse the value to an integer it will set the value (e.g. 000 => 0, 21b => noop).  The toShortMessage function now uses #rawErrorCode as this value will always be set appropriately.