Fix handling of px units in CSS hook setters.

turtle.animate { turtlePositionX: '+=100' } had been broken becuase
the CSS hooks for turtlePosition* were not handling things when
the units "px" were included in the set value.  This is now fixed,
and animate works again.  A test is added for this case.
Add sync function.

The sync function makes it easy to synchronize multiple animation
queues: by running sync(a, b, c), all the specified elements will
wait for each other to get to this point before proceeding.
Merge pull request #73 from PencilCode/sync

Add sync function.
Merge pull request #71 from PencilCode/hookfix

Fix handling of px units in CSS hook setters.
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.