HR Contract Wage Type

Reintroduce wage_type and wage_type_period classes that were removed from the OpenERP hr_contract official addon in 6.1.

These classes were present in 6.0 and removed in 6.1 following the introduction of the hr_payroll module. However, hr_payroll does not provide similar functionality and data cannot be migrated. This module restores identical functionality.

Module introduces also the hourly wage of the employee on its contract. This computed field remains invisible in screens but can be exported in CSV files.