[Warehouse] Prevent negative stock.

Task: https://accounts.openerp.com/web#id=10130&view_type=form&model=project.task&menu_id=3942&action=327
Pad: https://pad.odoo.com/p/r.KCBJWsVnxINg746v
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Allows you to prohibit negative stock quantities. 
This installs the module stock_negative.