[FIX] project, task, issue, portal: fixed access rules for employees/portal users

+ updated project.privacy_visibility help

- Public: everybody sees everything; if portal is activated, portal users
see all tasks or issues; if anonymous portal is activated, visitors see all tasks or issues;
- Portal (only available if Portal is installed): employees see everything;
if portal is activated, portal users see the tasks or issues followed by them or by someone of their company;
- Employees Only: employees see all tasks or issues;
- Followers Only: employees see only the followed tasks or issues; if portal
is activated, portal users see the followed tasks or issues.

bzr revid: tde@openerp.com-20130426144329-g9aahm295jqdgeyx