Added ResponseInterface

Hi Kris,

I've added `ResponseInterface` to the library and done my best to update the tests. It could possible do with another test. 

I'm also not sure about BC issues for people providing their own $response to `Browser::send()`. 

There are other mentions of `MessageInterface` which I could potentially replace as well. I've left the method signatures of the concrete clients (Curl/FileGetContents) and event listeners alone for now. But I could update those too.

Cheers