Finally managed to inject evaluation behavior into JAXB-generated classes by the cost of really complex and  unintuitive class hierarchy
Added custom MessageBodyReader for automatic schema validation
Restores all simple tests. Added helper class with factory methods.
Renamed expression element to calc. And changed class name to more meaningful CalculationRequest.
Also introduced new entity for server responses - CalculationResult
Renamed ExpressionMixin to Expression and placed it in restcalc.expr package
Changed Mixin suffix of extension classes to Ext. Moved these classes to restcalc.expr.ext package
Dumb JAXB!

I can't set separate schemaLocation for two schemas with default namespace.
I can't set globalBindings (choiceContentProperty in particular) for two schemas.
I can't run xjc twice from maven.

So I had to remove error element from CalculationResult class.
Updated WADL. Still can't understand how include schemas to WADL and generated client in full.
Split project up to client and server parts
Removed ResourcePlayground
Merge branch 'self-evaluating-expressions'