Release version 1.5.1

Release version 1.5.0

- Update to latest Timber version
- Allow null Throwables in log functions

Release version 1.4.0

- Add Timber.DebugTree() alias
- Update to kotlin 1.2 and timber 4.6.0

Release version 1.3.0

Release version 1.2.0

* Reduce method count with more aggressive inlining

Update dokka docs.