[WIP] Refactor product image

Hi, I refactor and simplify the module product image based on the new field ImageField
This merge depend of this one https://code.launchpad.net/~akretion-team/server-env-tools/server-env-tools

I will try to propose migration script in order to migrate the existing images

OLD MP : https://code.launchpad.net/~akretion-team/openerp-product-attributes/openerp-product-attributes-product-image/+merge/222397