Add “acts_as_emotion” and generated Emotion model

I didn’t like how we were using a *hidden* `Emotions::Emotion` model. In gems like [parole](https://github.com/mirego/parole) and [partisan](https://github.com/mirego/partisan), we generate an empty model on installation and we inject our gem behavior with some kind of `acts_as_…` method.

This allows the developer to easily add callbacks, validations, etc. to the model **and** is more consistent with our other gems.