Add Reference to Invoice, serialiser and deserialiser
Bring Payment object up to date
Add LineAmountTypes to Invoice output
Explicitly require dependent deserialisers
Add Payment deserialiser

* Includes trivial implementation of an Account deserialiser
Add the Payment serialiser to the Serialiser autoloaded-modules
Add PUT support to Endpoint
Add payments endpoint to Client
Make payment serialisable for the put_many situation
Add InvoiceNumber to invoice serialisation if it's there