Add format option to in_http. fixes #302

I implemented first version of `format` support in `in_http`.

This implementation doesn't support `field_name` option and assumes entire body is content like header provided json behaviour.

https://github.com/fluent/fluentd/blob/cd3e4461695543d16c18c0d7f873d0522506ee1e/lib/fluent/plugin/in_http.rb#L259

@kiyoto What do you think? Do we need `field_name` option?
Existence `default` behaviour can accept json / msgpack content without explicit format so need `json` or `msgpack` identifier in HTTP body.
But with `format` option, we already specified body format in configuration so `field_name` seems redundant.