Allow shop to be at base of url, not just a subdirectory
Currently, satchmo/urls.py assumes that you always want to put the shop under a subdirectory. If you want to put it at the base of the site, it just doesn't work properly.
In settings.py, if you have "SHOP_BASE='/shop'", then the url resolves to: http://yourshop.com/shop
if you have "SHOP_BASE='/'" then the url resolves to: http://yourshop.com// AND the url includes don't work.
This patch makes the Satchmo app "do the right thing" if SHOP_BASE is empty or "/" by simply including the satchmo/shop/urls into the base patterns in that case.
Reported by bkroeze