Customization

[dzhuang]

View customization:

1. Added required method create_instance_from_form for ImageCreateView.
2. Added required method create_cropped_instance_from_form for ImageCropView.

Allow customized image instance serialization with method:

1. For image url, introduced get_image_url
2. For crop url, introduced get_crop_url
3. Allow custom serialize method named serialize_extra (with request as arg) to do more serialization of the image instance.