Add unrepeats

Will allow users to unrepeat notices.

This has not been tested yet (since I haven't figured out how to get the unit tests running on my system), so it's WIP. Managed to get tests running, but they currently fail. Also will have to figure out how to get unrepeats working via the Twitter API.

Partially fixes #69 (closed).

EDIT: I have been unable to get unrepeats to federate, but they at least work locally. Perhaps someone else can try getting them working at a later date.

EDIT 2: Think I've figured out why things weren't federating properly. Gonna implement that fix.