add fluentd sink

This is a retry for #1 pull request.

fluentd[*1] is a open source log collector daemon written in Ruby. It supports several log transports using plugin architecture[*2] so that it can cover the main purpose of steno, 'multiple log destinations', such as mongodb, S3, cassandra, splunk, ...etc.

[*1] http://fluentd.org/
[*2] http://fluentd.org/plugin/