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.
This double loop and if makes the code far too indented and pretty ugly.

Consider using [itertools.permutations](https://docs.python.org/2/library/itertools.html#itertools.permutations)

Something like:
```python2
from itertools import permutations

for r1, r2 in permutations(hourly_rate_class.line_ids):
    if r1.date_end and (r1.date_start <= r2.date_start <= r1.date_end):
        return False
    elif r1.date_start <= r2.date_start:
        return False
```