idAttribute of a resource can be a computed property

[kenjiqq]

I know there is an issue about computed properties already, and they way it's supported right now is with the methods field. But it would be sweet would the possibility to use a computed property as the idAttribute for a resource, or maybe a method.

Some APIs i work with do not return a unique id field, their "id" is the combination of two fields(folder + nr in that folder for instance). I guess this would be easier when real computed properties are added.

[jmdobry]

This should be possible with how I'm thinking about doing computed properties.

This issue is dependent on #23.

I won't close it until #23 is closed, but keep it as a reminder to make sure the idAttribute of a resource can be a computed property.