Add descriptive_text as a settings type

Hi Pippin, 

This adds a new settings type called "descriptive text". This doesn't display any form fields, but does display the description as fixed text. E.g.

![image](https://cloud.githubusercontent.com/assets/1097338/4226020/affa0e9a-3933-11e4-8a70-95b363888494.png)

It can be useful to display information relating to settings in general without being attached to a specific setting field. The @since docblock will probably need updated.