Reimplement osx webcam module using AVFoundation.

Well, Apple deprecated QTKit in OSX 10.9. I'm pretty sure that is why we
ran into so many problems with threading... they must have changed some
QTKit internals (maybe reimplemented QTKit's interface using AVFoundation).
Regardless, we can no longer reliably call QTKit from a single-threaded
script. So I rewrote it in AVFoundation. This has a few downsides:

- 10.7 and up only. I used QTKit initially for its backwards compat,
but now realize that QTKit was 64-bit only, which limits you as well.
- No synchronous way to pull a single frame out of a video, or take a
snapshot, so I am relying on QTKit's (non-capturing) API to yank the frame.
This still appears to work fine.
- lots of work for joev :(