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.
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)