Errors include a statusCode

Sometimes it's useful to know the status code of a returned error. For example, if the error has a `502 Bad Gateway` status code you may with to retry the operation, however if the error is a `404 Not Found` you would not.

I've subclassed the `Error` class with an `HttpError` class, which has an additional statusCode attribute.

This is the first time I've written coffeescript, so please bear with me if I've made any style mistakes!

Thanks for a great library!