Description of the subject

I've just merge your current master branch with the feature added by [Bernat](https://github.com/bernat/timeline_fu) . From his description:

>>Little modification: Another field is added to the TimelineEvent table, the description of the subject. When the subject will be removed from the database, its description will remain in the event so the related past events can still be displayed and the deletion event can show the description instead of the subject itself (which is nil).