add member variables to Decode::$payload and Decode::$data

add member variables to Decode::$payload and Decode::$data.

your issue comment : https://github.com/Taluu/elephant.io/pull/3#issuecomment-51210483

`
I didn't add the data and the payload on the parent, because their purpose is different between the encoder and the decoder. It shares only the name of those two properties.
`

so
I think need to `$data` and `$payload` of `Decode Class`.