Add grouping and descriptions

- 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"
Add CHANGELOG file for keeping a record on changes between releases
Moving whitespace around for happiness
Merge pull request #24 from novoda/feature/grouped_tasks

Add grouping and descriptions for tasks, CHANGELOG
Update changelog.
Prepare for release v1.2.1.