title: Extend the onchange_employee_id method from hr_payslip class
body: <p>how do i extend the onchange_employee_id method from hr_payslip class, i have tried with this code :</p>
  
  <pre><code>import openerp
  from openerp.osv import fields, osv
  from openerp import api, fields, models, _, SUPERUSER_ID
  
  class hr_payslip(models.Model):
      _inherit = 'hr.payslip'
  
  
      @api.onchange('employee_id')
      def onchange_employee_id(self, date_from, date_to, employee_id=False, contract_id=False):
        *#my logic here* 
         return {}
  </code></pre>
  
  <p>and i got an error like this :</p>
  
  <pre><code>    Traceback (most recent call last):
    File "C:\Program Files (x86)\Odoo 8.0\server\.\openerp\http.py", line 544, in _handle_exception
    File "C:\Program Files (x86)\Odoo 8.0\server\.\openerp\http.py", line 581, in dispatch
    File "C:\Program Files (x86)\Odoo 8.0\server\.\openerp\http.py", line 317, in _call_function
    File "C:\Program Files (x86)\Odoo 8.0\server\.\openerp\service\model.py", line 118, in wrapper
    File "C:\Program Files (x86)\Odoo 8.0\server\.\openerp\http.py", line 314, in checked_call
    File "C:\Program Files (x86)\Odoo 8.0\server\.\openerp\http.py", line 810, in __call__
    File "C:\Program Files (x86)\Odoo 8.0\server\.\openerp\http.py", line 410, in response_wrap
    File "C:\Program Files (x86)\Odoo 8.0\server\openerp\addons\web\controllers\main.py", line 944, in call_kw
    File "C:\Program Files (x86)\Odoo 8.0\server\openerp\addons\web\controllers\main.py", line 936, in _call_kw
    File "C:\Program Files (x86)\Odoo 8.0\server\.\openerp\api.py", line 268, in wrapper
    File "C:\Program Files (x86)\Odoo 8.0\server\.\openerp\api.py", line 399, in old_api
    File "C:\Program Files (x86)\Odoo 8.0\server\.\openerp\models.py", line 5989, in onchange
    File "C:\Program Files (x86)\Odoo 8.0\server\.\openerp\models.py", line 5881, in _onchange_eval
  TypeError: onchange_employee_id() takes at most 5 arguments (8 given)
  </code></pre>
  
  <p>and iam using odoo v8, thanks before </p>