Allow subclassing ValidationResults.
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.

tristanpenman/valijson,tristanpenman/valijson