CVT support for Ken::Topic's

[michael]

For now Ken only returns simple properties. Support for so called mediator properties (aka ‘CVT’) should be added.

To be honest, I just don’t know how to wrap them appropriately using the existing Ken object model. Any API ideas are welcome, btw! ;)

E.g.

Ken::Topic.get("/en/bill_clinton").attribute("/people/person/education").values 
# => could return the concrete mediators either modelled as a Ken::Topic
# or using it's own Ken::Mediator class