Added way to check whether video stream is frozen

[gtmtg]

The video stream often freezes briefly, meaning that subsequent calls to ARDrone::getImage() will return the same image as before. I added ARDrone::willGetNewImage() to check whether we've received a new image since the last call to ARDrone::getImage(). This is useful in, e.g., computer vision, where it might be undesirable to process a frame that's identical to the one before it.

[puku0x]

Thank you gtmtg for your contribution !!