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.
Nitpicking, but `Check` would be better.
Imperative, present tense: "check", not "checked" or "checks".
Above you use "Get the hourly rate..." which is good. So for consistency sake.