Wrap log4go in a backend/log package

This should make it easier to swap `log4go` out for something else (or patch it).

Related to #260.
Referencing a `Global` object all the time is just tedious IMO. Let's create helper functions in the package that just call the equivalent method in the Global object. Again, wouldn't hold the PR back as long as a new issue number is opened for dealing with that.