Add TimeUnitValue property

[lchanmann]

TimeUnitValue is similar to UnitValue but for unitPropName of type 'foam.time.TimeUnit' with helper function to get milliseconds from the property.

Example:

foam.CLASS({
  name: 'MyData',
  properties: [
    {
        class: 'TimeUnitValue',
        name: 'expiresIn',
        unitPropName: 'timeUnit'
    },
    {
        class: 'Enum',
        of: 'foam.time.TimeUnit',
        name: 'timeUnit',
        value: 'HOUR'
    }
  ]
});

let data = MyData.create({expiresIn: 2});
data.expiresIn;  // returns 2
data.getExpiredInMs(); // return 7200000