Add return_image_data property to Stamp.

This allows the ReturnImageData property to be passed as a boolean
value, so that image data can be requested instead of the image url.
Add nonDeliveryOption param to Stamp.

Allow options to be specified for CP72 customs forms, which go in the 'Non
delivery instructions' box. This has to be one of the allowed
NonDeliveryOption values.
Merge branch 'return-image-data' into 0.4.0-improved
Merge branch 'non-delivery-option' into 0.4.0-improved