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.
Merge branch 'return-image-data' of https://github.com/freerunningtech/stamps into freerunningtech-return-image-data

Conflicts:
	lib/stamps/mapping.rb
Merge branch 'freerunningtech-return-image-data'