Added a globalListen function that adds a listener on every existing and future sockets

When I use socket.io, I often need to somehow add listeners to every existing and future sockets during my server execution. This is hard to do currently with socket.io, but it is a very useful feature that many users would probably like.

So I added a function in the namespace that does that. A new variable stores the global event callbacks so that each time a new client connects it can directly make it listen those events.

I also added two tests on the mocha file.

PS: Sorry if I'm doing something wrong, this is my first pull request and I have no idea what I'm doing.