Log: arg for caller must be 1, because default_logger is called via bblog().
t/Timeline.t: add a test to make sure filters do not modify the input original status objects.
t/Timeline.t: add a test for arguments given to filters.
Timeline: change spec and test. add_filter now takes the optional second argument $is_async. That way, users can always use add_filter() method for filters generated by factory methods.
Add a skeleton of App::BusyBird::Tutorial.