Minor changes to improve usability and robustness

I made a few minor changes, hope you like.

I also added some suggestions to the TODO list — I can add the `--uuid` option to `mate` if you think the cache can be made to use the document UUID as key — I am not yet familiar enough with the bundle, gist API, and desired workflow, to say how to best match online and offline documents, but I think the current scheme can be improved.