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.