Live stream feed

Added a StreamFeed class that provides asynchronous access to the video's frames.

So basically it's just a class that plays a video at a given framerate, and then you can query it for frames.
It's cool because it let's you see how well your code would work on a live stream.

For now, at least, it just plays the loaded video in a separate process, and so whenever you ask it for the current frame, it provides the latest frame from that other process. In the future, though, instead of just playing a video from file, the frames could come from an actual stream. That behavior can/should be refactored out at some point into subclasses, but for now there's no reason.