Fixed broken link generation in tasklistings

Problems: 
![bildschirmfoto 2014-10-24 um 12 21 02](https://cloud.githubusercontent.com/assets/485755/4768299/3f5923fe-5b68-11e4-993b-ce85f7cac5d1.png)

* Made link generation configurable and add tests.
* Dropped separate link generation helper for task listings and use `get_link` method on task object.

@lukasgraf @deiferni please have a look ...