use AV Foundation for capture on OS X

@calebjohnston's AV Foundation player has been integrated into glNext and this change removes the existing QTKit-based capture in favor of the AV Foundation-based capture implementation currently used for iOS which notably runs on OS X 10.7+.

*NOTE*: This was originally part of PR #408 but the [avf branch](https://github.com/cinder/Cinder/tree/avf) seems pretty dead at this point.