deviation_of refactoring

This pull request reactivates the latest changes to `#deviation_of` initially created by @jzernisch. `#deviation_of` is now returning the absolute distance. As a result of the discussion yesterday, I added two methods `#before?` and `#after?` so now it is possible to determine whether the time frame instance is before or after another time or time frame instance.

During development I used Guard so I also added a Guardfile to the repo. This is not directly related to the major topic of that branch, but it will be useful in future :shipit: