working osx with macbook

These changes got it working for me on osx.

`self._cv.read()` will sometimes return `False, None` in between having actual data ready. There is probably a better solution, and it may be related to osx not giving the actualy frame rate of the camera, but this gets the program up and running.

Great work, this thing is pretty neat.

![tumblr_msegnp5d9k1rw5vcwo1_400](https://f.cloud.github.com/assets/678338/1452134/1fc5ef70-42c0-11e3-8222-894b4a308235.gif)