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).
Because an employee is paid at different hourly rates whether he works on a job position or another, we need to differentiate how much time he spent on each job. An example is an employee in a restaurant. The same person can work as barman, waiter and busboy in the same working schedule and be remunarated differently according to each job.