Bugfix for multiple K4W or 1473 devices needing keep_alive. Opening by PID fails when multiple identical devices are on the system. This PR uses libusb_get_parent and lib_usb_get_bus_number to get the correct audio device for a camera. PR is backwards compatible with older versions of libusb.  Signed-off-by: Theodore Watson <theo@openframeworks.cc> (ofTheo)
Merge pull request #384 from ofTheo/bugfix-better-keep_alive

Bugfix for multiple K4W or 1473 devices needing keep_alive.

Reviewed-by: Benn Snyder <benn.snyder@gmail.com>