feat(angular.extend) - Simple non-breaking change to support deep extend

People use `angular.extend()` as a utility function, whether or not it's designed for that purpose. It can easily be extended to support recursive merging with very very minor additions. Developers coming to Angular expect `extend()` to support this.
If `dst[key]` is anything other than an object, we really need to replace it with an object --- I think `copy()` will use an Array if the source is an array, too.