Fix: create pdf previews during import#upload; test, too

Squashed commit of the following:

commit 42f35fc34d32a14c782548c1ea9b878b05e184fb
Author: Thomas Schank <DrTom@schank.ch>
Date:   Fri Apr 4 15:03:17 2014 +0200

    Require only 5 pdf-previews

commit d37071f78fdd0850747d4823d2d00770feaa6273
Author: Thomas Schank <DrTom@schank.ch>
Date:   Fri Apr 4 13:42:44 2014 +0200

    Fix: create pdf previews during import#upload

commit 41f3308ad092f2c07f4fd2e426fa38527248ba05
Author: Thomas Schank <DrTom@schank.ch>
Date:   Fri Apr 4 13:38:46 2014 +0200

    Added spec for importing a pdf