Turn Jently into a daemon. Distributed it as a RubyGem. 

Fixes #34.

Per #34, the main benefits of this PR are:

- Setup is now `gem install jently` + setup a config file + start the Jently daemon.
- Users can run multiple instances of Jently pointing at different repos by firing up multiple Jently daemons.
- Users can deploy Jently with configuration management easily, by using and adapting the shipped Puppet module.
- Contributors to Jently no longer need to worry about accidentally committing and publicly pushing config.yaml.erb into their fork.

There's a lot of changes to digest, so I'm looking forward to any feedback you have. 

I didn't tackle a full on refactoring of Jently as I wanted to keep as much of Jently the same as possible. I may submit another PR in the next few weeks to make the code a little more consistent. 

@vaneyckt once you sign off on the PR and merge, you can do the honours of releasing to RubyGems.org.