Add fluentd input plugin

The patch adds fluentd input plugin that supports fluentd's PackedForward, Forward and \[tag, timestamp, msg\] formats (by means of fluent codec) and is able to respond to udp and tcp heartbeats from fluentd agents.

Regards,
Sergey
>> More generally, is there ever a case where you want to ignore the tag?

I'd like to ignore the tag if an original fluentd event contains tags field, just like
```ruby
{"message" => "Hello World", "tags" => ["syslog", "fluentd"]}
```
So, by default the fluentd's tag is included into the logstash event, but if the user would not like to include it, like in the example above, he can disable tag inclusion by means of ignore_tag option.