http.Client.prototype.request is deprecated

[raineorshine]

I ran into some trouble using this module since it relies on a deprecated version of the node core http module.

Line 377 of etsy.js:

        request = http.Client.prototype.request.call(
            this._client,
            http_method,
            request_url,
            headers
        );

This needs to be updated to use http.request of current node. I'm sorry I don't have time for a pull request, but hopefully documenting this will at least some others some time if they are running into the same problem!