Fix events.list() and add support for count/offset

Hi, 
Thanks for writing this library! 

I fixed a small bug with the url in events.list (should not have a trailing slash) and added count/offset support as specified here: https://stripe.com/docs/api#list_events

````
list: function (count, offset, cb) {
    var nArgs = normalizeArguments(arguments);
    get("/v1/events", { count: nArgs.count, offset: nArgs.offset }, cb)
}

````