Storescrapper

* Improved logging behavior under sync and async environments

* Scrapping scripts must now provide the product types they can handle,
this simplifies the calling of the most common methods of the API.
* Included scrapping scripts now follow the requirement above.

* Changed signature of the store.products method. It now returns a list
instead of a dictionary. If a product is unavailable then its specific
price is set to None.

* Changed signature of the store.products and store.retrieve_product
methods. When dealing with unavailable products, the retrieve_product
method now returns a Product object with None as price. The products
method still returns a dictionary, but now its values are never None.
If a product is unavailable then its specific price is set to None.

* Changed signature of the store.products method, it now returns a dict of
url => product. This helps determine if a product was available or not
by the caller (the value of the dict is None for those urls)

* Fixed bug that raised an error when trying to import all stores included
in the package