Dirty checking support for Object.create(null) bindings

With this changes, objects initially created with Object.create(null) can be bind to the view. This is because if the valueOf function is not available, it will use Object.valueOf.apply instead
Can you cache `Object.prototype.valueOf` as `valueOfObject` in src/Angular.js instead, and just always use `newValue = valueOfObject.call(newValue);` ? thanks