Handle integer and datetime types

[ArnaudBuchholz]

Part of #271

**As a developer, I want the serialization functions to handle integer and datetime types **

Acceptance criterias:

• gpf.serial.integer
• gpf.serial.datetime
• a converter callback can be provided in the from & to functions to serialize / deserialize types. it receives the property definition, the actual value and the member name. It is invoked with the initial object as this.