Added a callback 'on_finished' for the video widget. Used in the same way
as other callbacks for other widgets. The callback is callable each time
you play the video file.
Forgot to import Weakmethod class from widget. Now importing correctly.
Changed the name of the on_finished callback to on_finish to keep in style
with other widget callbacks. The on_finish callback is now called
everytime the video is played.
Merge pull request #4 from nickpolet/master

Video.on_finish callback