Ready for review.

The module have receive a big refactor, in order to work without "shopinvader.variant"

Now urls

• are generated only when the method "_update_url_key" is called (so it should be called before the export)
• a flag is set on the record if something have changed to avoid useless recomputation
• url key are global by default (same for all website), but the it's easy with an extra module to change this behaviour

The module shopinvader_product_url generate the url for the category and the product

TODO

• add the redirect from product to category when deleting a product

Resolves #1373