Add crude publish_at feature; allow scheduling of slides

This changes the "publish" behaviour to be in line with the "available on" behaviour or products. 

Where "published" is not a flag, but rather a date after which the slide will be published. 

By leaving the date empty, a slide won't be available.

![schermafdruk van 2014-06-19 23 26 16](https://cloud.githubusercontent.com/assets/77059/3333917/c10db6ae-f7f8-11e3-8191-5d8974e32834.png)