Currency: Behavior change for to_json for demurrage currencies.

Currency#to_json should output a format that can be successfully converted back
to a Currency object via from_json. So for demurrage currencies it should output
the full hex currency code.

Currency#to_human on the other hand should always print something a human will
understand. For demurrage currencies we'll print the three-letter code for now.
Amount: Support passing demurrage currencies to Amount#parse_human.

If a reference date is provided as an option, the method will apply the
demurrage/interest rate compounded to that date.