Fixed definition for getProductImages.

The "id" parameter was missing, resulting in a 404.  This was added
using the same definition as getProductMetafields, although I'm not sure
that the type should be a string.

While fixing this I also corrected since_id and fields to be included in
the query not the uri.
Merge pull request #6 from elyobo/fix-get-product-images

Fixed definition for getProductImages.