Replace logging from Log class with Timber

[hieuwu]

With Timber we can log with current scope of the class. This helps find problem quickly and also make the log only available in DEBUG build.

[kirillt]

@hieuwu I remember considering Timber a while ago, but here are reasons why we haven't proceeded with it previous time:

1. Finding the problematic class by log message is not that difficult task.
2. Just filtering logs out from production build can be done using 4 lines of ProGuard.
3. Extra dependency and minor refactoring needed to replace logs.

Here is a really cool use-case for Timber though:

• In a release build, send errors and warning to our server (we must watch that logs on this level don't contain sensitive information).

But if we install a metrics server, e.g. AppDynamics, do we really need Timber to send errors there? Or AppDynamics can do it alone?