add format raw

I prosed to add `format raw`. 

Currently, even if users do not require to parse log lines, we have to write configuration like

    <source>
        type tail
        format /^(?<message>.*)$/
        ....
    </source>

This requires to parse log lines by a regular expression, and its overhead is not trivial. 

           user     system      total        real
    asis:    0.080000   0.000000   0.080000 (  0.081076)
    regexp:  7.590000   0.000000   7.590000 (  7.592521)

cf. http://blog.1q77.com/2013/01/fluent-agent-lite-%E3%81%A8-in_tail/

This plugin enables to write configuration like

    <source>
        type tail
        format raw
        messag_key message
        ....
    </source>

(where `message_key` is set to `message` as default) and does not require to parse log lines by a regular expression. 

There is a plugin [fluent-plugin-tail-asis](https://github.com/yteraoka/fluent-plugin-tail-asis) which does the same thing with this patch, and downloaded 2354 times as in 2013.09.02 http://fluentd.org/plugin/. It tells us that some users are already using this feature, and I propose to have this feature in the fluentd itself.