Fix the bar width

Remove typo that cause the bar width to be incorrectly calculated.

	modified:   src/histogram.js
Fix histogram updating and bar selection

The histogram supports multiple histogram instances which may contain
different configuration settings.

* Fix histogram sizing after the configuration has changed.
* Update the axis when drawing new bins
* Fix the bar selection to select the correct bins.
* Rename `config.color` to colors to specify that this allows an array
  of colors.

	modified:   specs/histogram_spec.js
	modified:   src/histogram.js