Added d3_demo to scratch dir, working on bubblechart.html going through mbostock tutorial on bubblechart.

Currently working.
Use linear scale to scale radius
Change src of d3 to local, not cdn. Declare all vars first in script. Playing with legend
Got legend labels and circles to work by using function(d) to set x and y coordinates - working
Moved d3.js and data_vis.js into static file, add url_for in html to indicate src. Working svg on index.html

 - Removed "libs" dir, moved d3.js to static
 - Used jinja url_for to refer to static directory
 - Moved data_vis.js to bottom of body, to execute after DOM loads
 - data_vis currently executes on load
Position test circles along x axis according to value position in array
Set radius and cx to be proportion to d and to width, respectively
Added attribute to circle for varying color by d
Add svg text elements, styled, to center of x position (middle of circles)