Add support for cancellation token.

[marcoregueira]

Here is my pull request for issue #1, adding support for cancellation tokens.

Example

            var tokenSource = new CancellationTokenSource();
            tokenSource.CancelAfter(5000);
            decoder.OnFrameReceived += Decoder_OnFrameReceived;
            decoder.StartDecodingAsync("http://83.128.74.78:8083/mjpg/video.mjpg", tokenSource.Token);

I hope you find it useful.

[4egod]

LGTM