fixes and improvements

[cahein]

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.

[futhr]

+4 for adding specs.

[cahein]

changed the gemspec to use spree 2.4 test still succeed

Note: the test_app task generates the following line in all.js and all.css: *= require spree/frontend/spree_product_zoom which fails, because there is no such asset

the install_generator adds *= require spree/product_zoom which is working

to fix this problem, the js and css assets could be moved and renamed

[cahein]

moved the style and javascript assets and changed the install_generator accordingly fixed the spec_helper to copy required test assets to the dummy app