Don't parse the response, not everything is JSON. Fix README

Hi,

I am using your library for some project. It did not work with simple String-messages. (I only send simple strings via websocket), so I removed the JSON parsing, which seems to be ugly anyway (according to your comment).
I think the one receiving the data should be responsible for parsing it.

Also in your README, the "onMessage" had two parameters, which is wrong.

Feel free to adopt the changes. It's not perfect I think, because I receive a strange character before my actual message string, but I have to investigate that further.