add ltsv format support to parser

Add Labeled Tab-separated Values (LTSV) format support to parser.rb. LTSV format is a variant of TSV. Each field of LTSV has a label separated by ':'.

The log line of LTSV is like this:
```
time:28/Feb/2013:12:00:00 +0900	host:192.168.0.1	req:GET /list HTTP/1.1	status:200
```

The configuration of in_tail with ltsv format is like this:
```
<source>
  type tail
  format ltsv
  time_format %d/%b/%Y:%H:%M:%S %z
  path /var/log/nginx/access_log
  pos_file /var/log/nginx/access_log.pos
  tag nginx.access
</source>
```

* time_key and time_format can be specified like JSONParser.
* delimiter and label_delimiter also can be specified.