add appearence delegate

Add a property selectedPhotoUrls for the GCImagePickerController, and add a protocol GCImagePickerAppearenceDelegate, It can customize the imagepicker navigation bar items. See sample code.

![Custom-navigation-bar-items-1](https://f.cloud.github.com/assets/506125/64874/093d23ea-5e5f-11e2-9679-8c2ffdc77f3b.png)

![Custom-navigation-bar-items-2-and-selectedPhotoUrls](https://f.cloud.github.com/assets/506125/64879/665c544c-5e5f-11e2-90e5-e5d697ed0298.png)