New camera example

Add a new example showing the webcam feed in the canvas. This example requires OpenCV (Python 2 only AFAIK). If the library is not available, a dynamic white noise is shown.
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> I think for the example you can just assume the user has OpenCV, and list it in the top docstring as a requirement. It's actually better in my mind to have them get an immediate error if they don't, since they might miss the print statement telling explaining why they get a blank screen. So maybe still catch the exception, but then throw another exception with a nicer message? (Should also be except Exception so that SystemExit isn't caught by this code)
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> What should Travis do in that case (we probably don't want it to install
OpenCV just for this example) ?