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).
I do not think it's overkill, I kind of prefer it, but OCA conventions and uniformity are more important than personal preferences. That's why I was aking if there is one.

P.S: try to not reply via mail or at least delete the quoted part, it's cluttering up the discussion page.