Add Video sprite.

For example, v = new Video makes a new video capture turtle. It
can capture the webcam (if allowed) by saying "v.capture()".

The Video constructor takes an options object that is translated to
the attributes of the video tag.

For manual control:

v.capture() starts capturing video
v.cut() stops capturing video
t.wear(v) makes a copy of the current video frame and puts it in turtle t.
v.on('play', fn) calls the function when the video actually starts playing.
Rename Video to Webcam.

Webcam starts capturing automatically so that there is no extra step.
Merge pull request #72 from PencilCode/video

Add Video capture sprite.