Add Logger Interface?

Some folks on Twitter mentioned regularly setting up a logger (Timber is really popular in the Android space).

Given this, it might make sense to add some default logging implementation. I think it's okay if we add Timber, but I also think it might be worth putting behind a wrapper interface? This way we could also supply our own `FakeLogger` implementation.

Not sure if that's overkill, looking for feedback. Will put up a suggestion PR soon.