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).
Hi Pedro,
I supervise the work of David on this R&D project.

The point is that an employee can have several jobs with a specific hourly rate related to each job. (Job 1 = 10 $ / hour, Job 2 = 15 $ / hour, etc...)
The best example is in the building industry : a worker can do electrician work, plumbing work, etc... When he fills his timesheet, we need him to specifiy how many hours he worked as an electrician, etc...
As a matter of fact, we can know his salary base for the payslip period by doing the calculation by job * specific job hourly rate.
As Odoo is using analytic accounts for the timesheets, with the architecture we have chosen with David, an employee can specify a project (analytic account) on his time sheet and then select an specific activity (job) for which he want to enter hours.
ONce the timesheet has been validated, we can import the hours from the timesheet to the Payslip preparation, with the job detailed hours to make the real calculation of the payroll possible.

Please feel free to ask if you have any further Question, Pedro.

Pierre