decouple the default signalling server url and add it to instantiation

I plan on submitting the library to [cdnjs](https://github.com/cdnjs/cdnjs) and I think it would help if we would emphasise the requirement for a [signalling server](https://github.com/andyet/signalmaster) and that this implementation uses [socket.io](https://github.com/learnboost/socket.io).