refactors stats and optimizes js

JS libraries for stats (d3, c3, etc.) only load now when needed (stats/show)

app/assets/javascripts/stat_graphs.js

@@ -0,0 +1,15 @@
+//= require d3
+//= require c3
+//= require c3ext
+//= require stats
+
+var initialize_stats_modules = function() {
+  App.Stats.initialize();
+};
+
+$(function(){
+
+  $(document).ready(initialize_stats_modules);
+  $(document).on('page:load', initialize_stats_modules);
+  $(document).on('ajax:complete', initialize_stats_modules);
+});