Product clone - optional image cloning

Cloning a product currently copies the images as part of the process. This submission makes image duplication a configurable option, which is enabled by default for backward compatibility.

The impetus behind this change is that products with large # images can take a very long time to clone. Given that you won't likely want the same images for 2 products, this allows you to turn off image duplication if your business process does not require it.

This configuration flag is set to true by default, which out of the box leaves the product clone functionality identical to how it works today. 
In order to test this change, you'll need to set the configuration flag to false. 
Recommendation is to do the following in under config/initializers/spree.rb in your rails app:
```
Spree.config do |config|
  config.dup_product_images_on_clone = false
end
```