allow resources to be a prefix

Well, sometimes people may need server resource to be a prefix, like `/socket.io/v1.0/`, where `v1.0` might be handled by any framework URL router and passed to appreciated socket.io namespace. So our  server initialization looks like:

    address = 'localhost', 9100
    server = SocketIOServer(
        address, application,
        resource='websocket', # instead of resource='websocket/v1.1',
    )

There was a [merged ticket](https://github.com/LearnBoost/socket.io/pull/744) on socket.io that make it possible to use a regexp on resource URLs. But since *the implementation was hard to explain for me, I thought it might be a bad idea*. So I came up with a one line modification that it was damn easy. 

I'm curios about what people say if it should be regex allowed or this `.startswith` solution is just fine.