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.
That said, I really think we should only be taking this path if `isObject(obj) || isFunction(obj)` --- it doesn't really make sense otherwise