PS Move API 3.1.0: Support the PSEye on OS X via PS3EYEDriver

Add support for grabbing frames from the PSEye camera by using the
PS3EYEDriver and libusb on OS X. This needs to be enabled separately
in CMake. By default, it tries to find libusb from the system (via
pkg-config), but there's also support for building libusb-1.0.9 (in
external/) statically and linking against it statically.

Static linking is also what contrib/build-osx-snapshot uses now. This
means that the iSight isn't supported for those snapshot builds, you
would have to disable the configuration options in the script and
rebuild without PSMOVE_USE_PS3EYE_DRIVER (you can also skip building
libusb in this case).

The frames read from the USB camera are encoded in YUV422 format, we
currently convert this using a function from PS3EYEDriver, modified to
output BGR data instead of RGBA. By using multithreading and better
buffer management (or buffer management at all), the frame rate could
definitely be improved, but this is not implemented yet.

Tested with OS X 10.9.1, but older releases should also work.