Add company configuration for "Amount payment for working holiday"

Add a configuration field for the amount payed for a working holiday.

Remove value "2" from the code on _working_holiday function (payroll class).
code:

  working_schedule = self.pool.get('hr.payroll.cr.working.schedule').browse(cr, uid, contract.working_schedule.id)
  pay_per_day = ((base_salary/30) / working_schedule.hoursperday) * 2 #TODO: change the 2 value for the default value in company
  for wh in working_holiday: