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.
You misunderstood me in that last commit --- I mean on line 353, the condition should not be `if (obj)`, it should be `if (isObject(obj) || isFunction(obj))`