Timber.Tree.log() param documentation is incorrect.

[vaughandroid]

The documentation of the message param indicates that it may be null when t is not null:

@param message Formatted log message. May be {@code null}, but then {@code t} will not be.

However, the implementation of Timber.Tree.prepareLog() will always append the stack trace to the message when t is not null. Hence, message will never be null.

[vaughandroid]

Did a quick PR for this: #207