Role descriptions for display in the sharing view overlay

:construction: includes #39 

This PR adds support for role description texts, which are also displayed role description overlay of the sharing view.

e.g.
```rst
    editor-in-chief role description:
      The editor-in-chief reviews and publishes content.
```

![bildschirmfoto 2014-05-07 um 23 48 56](https://cloud.githubusercontent.com/assets/7469/2912584/e6e8120e-d67c-11e3-94e4-2cb80adb3d46.png)

This allows to add additional explanation of how the roles work and how they are meant to be used for the users which are allowed to grant roles. It is optional.
The texts are included in the automatically generated translations, so that they can be translated into other languages for multilingual sites.