Add withdrawn and fix time serialization to conform to the schema.

[calebbrown]

• withdrawn timestamp does not exist in the model - add it to the model.
• fix the timestamp serialization to ensure published and withdrawn are only present when the value is not zero
• fix timestamp serialization to use RFC3339 for JSON (it uses RFC3339Nano by default)
• fix the timestamps to be normalized to UTC as per the schema

[G-Rath]

Note this will conflict with #423

[calebbrown]

Note this will conflict with #423

Thanks! Fixed :)

[oliverchang]

thanks Caleb!