Allow subclassing ValidationResults.

I use this because our Path constraint needs information global to the current validator(the root of the document).
The natural place for this would be in a subclass of Validator, but by the time we are in the validation visitor we don't have access to that.
I prefer not to use a global because that would introduce threading issues.  That leaves putting the state in the results.

include/valijson/validation_results.hpp

@@ -19,6 +19,7 @@ class ValidationResults
 {
 public:
 
+    virtual ~ValidationResults() {}
     /**
      * @brief  Describes a validation error.
      *