label on color swatches

[Fil]

closes #834
supersedes #621

[mbostock]

Are the other changes necessary here to support the label? Can you explain those a bit?

[Fil]

When there is a label (which now defaults to the scale label), the swatches (color rect + color label) are now wrapped into their own div (.plot-swatch). We don't wrap if there is no label.

The div containing the swatches still gets the proper .plot-swatches-columns or .plot-swatches-swatches className depending on the columns option, the only change here is that it is decided a bit later in the code.

The CSS ensures that the label is close to the swatches (also is a bit simpler than what we had before), with the same vertical gap between them as with the ramp.

Here are two representative examples:

"bad"	"ok"

I suppressed the automatic label on some of the charts, either when it didn't seem useful (because it would be obvious from reading the tick labels), or simply for aesthetic reasons.