Histogram (#888)

* Add Histogram as a visualization

The css and js file use the histogram code from https://bl.ocks.org/mbostock/3048450.
THe viz.py extends from BaseViz to create chart data only for one histogram

* using d3.layout.histogram

* CSS updated

The new css has been used from the d3 chart http://bl.ocks.org/mbostock/1933560

* bars are visible

* added semicolons

* histogram from http://bl.ocks.org/mbostock/1933560

It takes as input no of bins. The histogram cycles through
a set of colors for different lengths of the bar. It places a
y axis coordinate on top or on the upper end of the bar
whichever is suitable.

* update style changes

caravel/assets/javascripts/modules/caravel.js

@@ -32,6 +32,7 @@ const sourceMap = {
   cal_heatmap: 'cal_heatmap.js',
   horizon: 'horizon.js',
   mapbox: 'mapbox.jsx',
+  histogram: 'histogram.js',
 };
 const color = function () {
   // Color related utility functions go in this object