Add Payment end to end identifier

[mortondaniel]

Currently the Retrieve Transaction API supports returning:

• rawText
• description
• message

However if a transaction occurs that is a transfer from another person, and they only entered a reference id, and didn't populate the full text description, then both description and message are null / empty.

And rawText appears to be set to the payer's name.

It would be great if the reference could be sent through on transactions from this API as well.

[crbon]

Please fix this. Should be a pretty small thing :)