Allows to add to basket a product directly from thumbnails blocks

I've added this feature to add a product directly from thumbnails blocks:

![capture decran 2014-03-27 a 15 30 11](https://cloud.githubusercontent.com/assets/103900/2538369/d8608c06-b5bc-11e3-99ff-a3d743f7705b.png)

If product is a master product (has variations) or has no stock, then the "Add" button is transformed into a "See" button that redirect on the product page.