NetworkClient request to return the URLSessionTask

[markst]

Related to issue #96. I'd like to be able to cancel the network request.
If the NetworkClient protocol function were to return the URLSessionTask performed:

func request(method: String, url: URL, payload: Data?, callback: @escaping NetworkClientCallback) -> URLSessionTask

It would be possible to cancel the request upon cancelling the operation:

override public func cancel() {
    super.cancel()
    self.networkTask?.cancel()
}

[markst]

Happy to create a PR if you think this is a good idea.