I have changed MessagePack from JSON in serialize format.

I test in mac OS X in local fluentd.
I test the following code:
```ruby
log = Fluent::Logger.new(nil, :host=>'127.0.0.1', :port=>8888)
log.post('debug.forward', {"agent"=>"foo-msgpack.to_msgpack"})
```
This succeed.

fluentd log in stdout:
````
2013-08-10 18:01:05 +0900 debug.forward: {"agent":"foo-msgpack.to_msgpack"}
````