Add module hr_contract_hourly_rate

This module adds hourly rate classes on contract jobs. When computing the employee's payslip, this will allow to fetch the employee's hourly rate for the payslip duration. The method get_job_hourly_rate on contract model serves this purpose.
@bwrsandman Yes it will, but that's the point. If the user needs two rates, he must put a end date to one of these. Example, between 2014-01-01 and 2014-12-31, you earn 25$/hour and from 2015-01-01 to infinite, you earn 30$/hour.