Added a parser for "Labeled TSV"-formatted values

You can see what "Labeled TSV" is at the blog posts below:

  * http://stanaka.hatenablog.com/entry/2013/02/05/214833
  * http://d.hatena.ne.jp/tagomoris/20130206/1360155675

In this pull request, I added a `LabeledTSVParser` and confirmed fluentd worked well with such a config below:
```
<source>
 type   tail
 format ltsv
 path   /path/to/log
 tag    debug
</source>
```