Fix #28: Add buffered recording to base class

This change adds new properties ::buffered and ::num-buffers to the base class.
If ::buffered is TRUE, uca_camera_start_recording will spawn a new thread which
will call the camera-specific grab. Any call to uca_camera_grab will return the
next item from the ring buffer.
Merge pull request #31 from ufo-kit/add-frame-buffering

Add buffered recording to base class