[FIX] hr_payroll_account: do not create entries with amount at 0

If the salary compuation gives a salary of zero, skip the creation of the account.move.line (not good to have lines at 0).
Fixes lp:1298116, opw 605816
[FIX] purchase: minmum purchase quantity and float precision (opw 610557)
Merge branch '7.0' of https://github.com/odoo/odoo into 7.0-win32