Issue 1  convert to polyfill

It's not a complete re-desing, but only some tune-ups to help compatibility and improve of my DataChannel-polyfill. All the management to work as a polyfill is there (https://github.com/piranna/DataChannel-polyfill/commit/bf63d0d684e70a6077e901dc3bfc9e56fa06653e), although would be interesting to be integrated here and make 'reliable' an independent polyfill (tell me if you like the idea and I'll do it. In fact, the code will be cleaner in both projects...).

The mayor downside is that the 'onmessage' method (now a EventListener) is having the message inside a 'data' attribute ({data: <message>}) instead of being directly served (<message>) as the specification says.