added activity module

This module implements the activity model. The model will be used in payroll and timesheet modules to identify what an employee did in a worked day or a timesheet record (he worked on a job position or took leave).
Another example is specific to Quebec, there is a contribution of the employer called CNT. For its computation, it is required to know how many hours an employee spent on a job on which he supplied heavy equipment (like trucks, tractors, loaders...) at his own cost.