LabeledTSVParser cannot handle time_key and time_format

LabeledTSVParser cannot handle `time_key` and `time_format` expectedly.
Without this fix, ValuesParser#configure will throw following ConfigError.

    2013-02-14 21:03:45 +0900 [error]: config error file="fluent.conf" error="time_key (\"time\") is not included in keys ([])"

I used following fluent.conf for my tests.

    <source>
      type tail
      path /var/log/nginx/ltsv.log
      pos_file tmp/ltsv.log.pos
      tag nginx.ltsv
      format ltsv
      time_key time
      time_format %d/%b/%Y:%H:%M:%S %z
    </source>