Add product_code_mandatory and product_code_unique

This PR supersedes the merge proposal made on Launchpad some time ago by @lbellier: https://code.launchpad.net/~numerigraphe-team/openerp-product-attributes/7.0-add-bremskrel-modules/+merge/219858
"I propose these 2 new modules :
product_code_mandatory: all is in the module name.
product_code_unique: having a unique product code, without sequence.
These modules have come from BREMSKERL branch (they are OK to merge).
Loïc BELLIER"
I prefer a 'read' to avoid reading the full recordset of product.product