Lazy Logging with lambdas

Given we have the need to write something like:

```
Timber.d("Something was done: " + foo + " and " + that + "with " + expensiveOperation());
```

It would be nice to avoid calls to `expensiveOperation()` when no logging is happening _(e.g. when "forest" is empty, etc.)_.

Since now [lambda expressions are supported in default toolchain](https://developer.android.com/studio/write/java8-support.html), the syntax could be like this:

```
Timber.d(() -> "Something was done: " + foo + " and " + that + "with " + expensiveOperation());
```

For example, it's implemented in [Log4j 2.4](https://logging.apache.org/log4j/2.0/manual/api.html).

Start by reviewing Timber's public logging API and how its current methods are invoked. Compare the proposed lambda form with the existing API and the linked Log4j example; done means expensiveOperation() is not evaluated when logging is inactive while the syntax remains compatible with the supported Android toolchain.

Written by the indexing model from the issue text.