Inheriting specifications #88

In case we have classes that share similar behavior, it would be nice to specify it only in one place. For example, if we have inherited classes.
I think the most natural way is to inherit the specification. To make it work we need a slight modification in the way the context of the example is determined.  If we use declaring class of the method, then  we incorrectly get parent class. So, first we try to get the class from specification. If it does not exist, we fallback to the class which declared the method.