Add automation to twitter usage

Right now, all the twitter stuff is managed by @sims1253, so to take some work from his shoulders, there should be a bot that will automate to some extend interaction with twitter.
The goal is that after merged PR, someone from specific (configurable) team can comment something like:

```
Twitter: did a great job! Now coala support new great feature.
```

The bot should pick that up, look up author(s) of commits from PR, based on Github username look up Twitter username, look up relevant commits and possibly add the link. From all this information create and post to twitter, mentioning the author. Maybe it'll be also great if it could pick up related hashtags and add them automatically as well.

If you'll have any ideas on how to automate either managing of coala or twitter interaction, or what features this bot may have, feel free to share. Also, if there is already something similar, it would be great to know.

If you're planning to work on this, please ping @sils1297. He may share some code with you for interaction with Github, so you won't have to build this from ground up.

Start by clarifying the proposed workflow: a configurable team member comments on a merged PR, then the bot finds commit authors, maps GitHub usernames to Twitter usernames, and assembles a tweet. Ask @sils1297 about the existing GitHub interaction code; done would mean the bot reliably creates and posts the requested author-mentioning tweet, with relevant commit links or hashtags where supported.

Written by the indexing model from the issue text.