Added "Watch" functionality on issues. It allows users to receive mail notifications about issue changes.
For now, it's only usefull for users who are not members of the project, since members receive notifications for each issue (this behaviour will change).

git-svn-id: http://redmine.rubyforge.org/svn/trunk@453 e93f8b46-1217-0410-a6f0-8f06a7374b81