Fast Fill

Fixes #346 

There are two main "innovations" here:

  1. The chart fill is now drawn in a sane order (see my comment on issue #346 for a chart)
  1. I created an optimizing canvas renderer, which drops superfluous points on their way to the canvas.

I took screenshots of `tests/dense-fill.html` both with and without the optimization. There were a few differences with the antialiasing along the edges, but nothing visually distinguishable.

The performance improvement can be dramatic. For a 10,000 point series in a 480x320 chart (i.e. the benchmark), I see a drop from ~700ms/draw → 25ms/draw, which is roughly in line with the performance w/o `fillGraph` set.

The speedup is entirely explained by the number of line segments being sent to the canvas. `tests/dense-fill.html` draws a 10,000 point series on an 800px wide canvas. Before, it issued 40,000 `moveTo` and `lineTo` calls. After, it issues ~1490.

As it turns out, JavaScript is pretty fast these days!

That's changed substantially since dygraphs started. Working w/ ReactJS has really changed my mentality. Everything that's done in pure-JS land is fast. Everything that touches the DOM is slow.