Made '__base' method available for mocking

[L0stSoul]

I have two classes

     var A = inherit({
            m : function() {
                //some unknown dependencies.
                return 'A';
            }
        })

       var B = inherit(A, {
            m : function() {
                return this.__base() + 'B';
            }
        })

And when i want to test 'm' method in class B, i can't mock method 'm' in base class. So for test purpose i need to mock all deependencies that base method 'm' have. It's really inconvenient and hard to maintain.

this PR is first try to somehow fix this situation.

[dfilatov]

Thanks!