Added new notification framework

It is now possible to add notifications to
users and notifications will be shown in user's
profile page. There is also an indicator in the top
bar if there are new notifications.
Notification framework

Added also the new files which were forgotten to add to the previous commit.
Modifications to notification framework

Notifications are now bound to specific course instances.
Also the notification menu is now a bit different, it shows
a pop-up menu for selecting the course.
Merge pull request #137 from ttsirkia/master

Notification framework