Adds TwitterTweeter cog.

An initial implementation for ArchiveTeam/ArchiveBot#29. 

Usage: modify `twitter_conf.json` with the API keys and supply `--twitter-username` to active the cog.

The default time between Tweets is 120 seconds to avoid flooding a user's timeline and avoid the automatic user account suspension (as the URLs could be triggered as spam).

It uses Redis as the semi-persistent store and discovers jobs based on the update channel. Messages older than a month are automatically cleaned out.