Specify how to set a "system property" in the `README.md` file
 * Add `videoCodec`, `videoBitrate`, `audioCodec`, and `audioBitrate` properties to `FrameGrabber`
 * Work around `avcodec` and `avdevice` not loading properly for `FFmpegFrameGrabber` and `FFmpegFrameRecorder` (issue #24)
 * Do key frame detection in `FFmpegFrameRecorder` based on `AVPacket`, not `AVPicture` (pull #20)
 * Make `FrameGrabber.createDefault()` throw an exception on unsupported input, instead of returning a cryptic `null` (issue #30)