Implementation of the ticket system

The tickets will be the hearth of the projects, with the wiki. A ticket is a demand of an user to correct a defect, or to had a new function. Each ticket will have:
- a registrant;
- an assignee;
- a priority;
- a type (task, defect, enhancement);
- a status (opened, closed, won't fix, cannot reproduce, ...); it will be no difference between status and resolution;
- a component affected;
- a targeted milestone (optional);
- a summary and a description);
- a list of keywords (aka tags).

Tickets will receive comments (which are linked to changes to the ticket) and attachments, if needed.