fix to use fluent'd log event processing thread
to separate Engine.emit for $log from each plugin's main thread.

On current code, if Engine.emit on $log raises any exceptions, main thread of
each plugins (that calls logger) will fail and run into shutdown sequence.
This behavior is unexpected for many users.
Merge pull request #142 from tagomoris/log_event_thread

fix to use fluent'd log event processing thread