Decorator Pattern extend()

Hi Addy,

I wonder why in this line of code https://github.com/addyosmani/essential-js-design-patterns/blob/21346c134aa953ba469782372c90e42c36a1f083/book/snippets/01-JavaScript-Design-Patterns/12-the-decoration-pattern.es5.js#L255 we are extending `CaseDecorator` with `MacbookDecorator`. As functions, they don't have any enumerable properties by their own yet. Shouldn't happen the extension in the prototype like:

`extend(CaseDecorator.prototype, MacbookDecorator.prototype);`

because trying to do this `console.log(decoratedMacbookPro.addEngraving());` results in a `TypeError: decoratedMacbookPro.addEngraving is not a function`