Adds additional arguments to bind function, as the pusher bind supports it

First off, thanks for the pusher-test-stub api, made testing a lot easier.

Found that the bind function in pusher api allows to bind a hash in the context of callback. In our site we are currently using the bind function as below: pusherChannel.bind(currentPusherEvent, pusherCallback, {url: url}); the last argument {url:url} is made accessible in pusherCallback function as this.url. Found that this is missing in the pusher-test-stub code.