Faster, better animation

I just pushed a change to The Render Engine so it will use the browser's native requestAnimationFrame() call if it's available.  Currently this is an engine option and defaults to false.  You can see the difference by changing the option in the Vector demo's "engine.config" file to true. According to Paul Irish's blog post (http://paulirish.com) it allows the browser to optimize the redraws.  Also, if I read correctly, I can target the actual rendering context and it will only perform the optimizations for that context.  I need to test that claim, but so far, this is a nice change!