Add task for retabbing all files in project

Replace hard tabs with two character soft tabs.

lib/tasks/whitespace.rake

@@ -3,5 +3,8 @@ namespace :whitespace do
   task :clean do
     sh %{find . -name '*.rb' -exec sed -i '' 's/ *$//g' {} \\;}
   end
+  task :retab do
+    sh %{find . -name '*.rb' -exec sed -i '' 's/\t/  /g' {} \\;}
+  end
 end