* 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
