Fixes in properties evaluation and additional unit test cases

I've started to use json-molder and added/fixed a few points which occured:

+ If the `namespace` property of the model description is missing, the whole `origin` object will be used.   
+ Correction of the property evaluation if a `namespace` is given in the model description.
+ Added possibility to access already evaluated properties within computed properties.
+ Added unit tests to test property evaluation with a given namespace and for computed properties.