refactors stats and optimizes js

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

app/views/layouts/admin.html.erb

@@ -9,6 +9,7 @@
     <%= stylesheet_link_tag    "application" %>
     <%= javascript_include_tag "vendor/modernizr" %>
     <%= javascript_include_tag "application", 'data-turbolinks-track' => true %>
+    <%= content_for :head %>
     <%= csrf_meta_tags %>
     <%= favicon_link_tag "favicon.ico" %>
   </head>