Add the support of configurable product

Trouble:

No support of the configurable product (or let's say there is one, but not enough for large product base)

Solution:

One table per attribute set instead of all on the product.product table (see: https://blueprints.launchpad.net/magentoerpconnect/+spec/use-one-table-per-magento-attribute-set) and add the possibility to chose for every attributes if it is related to product.template or product.product.

Base this on product_variant_multi and make the wizard better for product creation. Products modifications will be much more easy, modifying an product.template attributes will modify every product !

Add the support of Configurable product in the Magento API's.