Replacement of ko.utils.extend in constructors with prototypal inheritance.

On my front-end stress test, `ko.utils.extend` is on top of the profiler(chrome 23, OS X). It was called 12000 times, mostly from ko.subscribable(that was called about 6000) which took 70ms. I think it would be faster if we use prototypal inheritance instead. I can't really take down my observables count, because all of them are needed.