Bugfix: Render axis below horizontal plots.

To reproduce: go to the vertical example, and change the orientation from
"bottom" to "left". Observe the axis tick marks above the plot, reaching down
into it.
Render measurements after (above) markers.

It's how Stephen Few wants it, from the design spec:
Whenever the featured measure intersects a comparative measure, the comparative
measure should usually appear behind the featured measure.
Inline default accessors to anonymous functions.

Also, no need to specify a transform that does nothing.

Mostly a style thing.
Add more classes to marker, range

If I have multiple markers, I want the second to be less prominent.
If I have charts with a varying number of ranges, I also need to know the total
number of ranges.
Don't always sort; allow explicit arrays.

Rather than call ranges, markers, and measures, see if they're a function first.
Sorting the markers is unhelpful: they won't occlude each other and it's helpful
to be able to style the first marker more prominently.
Sorting the measures is unhelpful for the same reason, we want to always know s0
is the actual value and s1 is the forecast. However, for rendering, we sort them
to prevent occlusion.
Add documentation to README.

Note that this documention reflects the changes I have made.