More flexible mail notifications settings at user level. A user has now 3 options:

* notification on any event on all his projects
* notification on any event on selected projects only (if the user belongs to more than 1 project)
* notification only for things that he watches or he is involded in (eg. issues that he watches or he is author or assignee)

git-svn-id: http://redmine.rubyforge.org/svn/trunk@855 e93f8b46-1217-0410-a6f0-8f06a7374b81