Added getSource function.  Just prints to console for now.
getSource function modifies 'Sources' box at bottom of page and its link
Merging from master gh-pages
Modified visualization.js to provide sources
Fixed bug where source was still showing after 'clear selection' was clicked
Merge pull request #74 from charlesdguthrie/gh-pages

#27 Source legend (lower-left) now shows name and link to source