Moved HTTP adapter into main blip package.

Also some bug fixes etc.
Fixed race condition handling incoming message
Improvements

* Message now has a Sender property. So when handling an incoming message
  from a listener connection, you can now get the Sender object to send
  your own requests.
* Sender.RemoteAddr() returns the IP address of the peer.
* Added more info to the README.
Workaround for HTTP body without Content-Length header.