Solves (almost) all TODOs from the original author...

And simplifies the watcher statement syntax, a lot. Now when someone do a `guard init markdown` the thing will just works by default.

To do the changes, I refactored the original project a lot and added some specs.

Besides that, this PR adds some sugar like changelog, .travis.yml and such.

I don't know if the number of commits will be helpful or harmful, but I'm here to do any changes that you guys need to pull this PR if you want. Just let me know.