tweaked/idealized uproxypeerconnection interface

Tweaked the uproxypeerconnection interface so my work on socks-rtc compiles and updated samples to match.

TESTED: grunt and ran `src/coreproviders/samples/freedomchat-chromeapp/` manually, messages still flow, no console errors. 
I've a feeling `signalMessageForPeer` would read better.

Then your code could read like:
```typescript
on('signalMessageForPeer`, (signal:WebRtc.SignallingMessage) => {
  signallingchannel.send(signal);
});
```