[FIX] hr_expense: computation of tax/base amount

When computing the amount for a tax (included) with children, accumulate the included taxes to avoid taking only the last one (opw 614867).
[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)
[FIX] product: name_search handles negative operators
[MERGE] forward port of branch 7.0 up to 9cc54dc