Added support for emitting to namespace

Not for merge yet.

This PR adds support for restricting your emits to a specific namespace. Requires changes to socket.io-redis to work (the last test that tests for not emitting to all namespaces fails). Answering issue https://github.com/Automattic/socket.io-emitter/issues/1

I will post more details on this PR when socket.io-redis is compatible and everything is checked to work.