Make it easy to include specific handle values in SOAP requests
Adds SerialNumber as a possible key/value for Handles

Used by Entry, CreditorEntry and DebtorEntry.
Factory method for creating new handles
Handle should know if it's empty
Tighten up entity equality specs
Handles aren't regular properties

They need to be handled on their own as they are composed of other properties
(and different properties for different entities at that).
Fix issue where only one line could be added to invoice