Always use respondsToSelector: to check if delegate methods are availabl...

Hi, Wesley, your project is great and thank you so much!  I found something can be improved.
You can pass `conformsToProtocol:` test by simply adding code like below, without implementing any method in the protocol.

    @interface WSMainViewController : UIViewController <WSAssetPickerControllerDelegate>

Suppose `WSMainViewController` does not have the method `assetPickerControllerDidCancel:`, and when the "Cancel" button is pressed, the program will crash.

So I think using `respondsToSelector:` to check if delegate methods are available is more reliable.