Add grouping and descriptions for tasks, CHANGELOG

(Resubmit of #23 to merge into develop instead of master)

![command-plugin-deluxe](https://cloud.githubusercontent.com/assets/45467/2971646/a68ed2ca-db6c-11e3-94b5-9d9c53df0145.jpg)

- tasks can have descriptions now, default tasks have a description set already
- all command plugin tasks will be shown under one headline rather than in "Other tasks"
- start CHANGELOG file for keeping a record on user-facing changes