Fix #6 - a slow poke for paused animations.
Fix #6 - a slow poke for paused animations.

In some browsers, requestAnimationFrame is paused in background tabs. This can
cause the page to hang when returned to the foreground, as when a huge queue of
chained transitions (d3/d3-transition#29 mbostock/d3#2211) builds up. So we now
wake up every five seconds to flush the queue.

d3/d3-timer