Merge pull request #592 from ostcar/overlay

New Overlay API
New Feature: List of speakers.
Norman, please review this commit

hide and show old speakers via JS
sort order of the list of speaker via JS
Updated template and some minor fixed for list of speakers.

Fixed missing line break. Updated projector slide.
fixed old speaker_list
used datetimefield for the right order of old speakers after midneight
Model documentation and some css
fixed ordering