fixes and improvements

Greetings,
using the spree_product_zoom extension, we found several problems.
Most obvious is the overwriting of the 'zoom.gif' below the main image (through showVariantImages in product.js.coffee).

Also, the behavior in different scenarios of product/variants was not like we need it.

Added a features test to document the current functioning of the extension. Unfortunately, the product images are not automatically uploaded yet and need to be copied into the dummy manually.

You need to remove this line it already set to 2.4.0, then we can create a 2-4-stable branch from this.