Use poll() to wait for capture with timeout

[leokoppel]

Before calling dc1394_capture_dequeue(), poll() for the frame to become available with a 500ms (arbitrary) timeout.

Fixes #48 (100% CPU use from constant polling), and is a different solution to #22.

It is the method "to cleanly stop a blocking capture" suggested in the libdc1394 FAQ which makes up for dc1394_capture_dequeue() having no timeout option.