add some indexes that speed up some queries

found with the help of query_reviewer at http://code.google.com/p/query-reviewer/
also fixed a removal of an index when you migrate back
you need to rake db:migrate to create the indexes

db/migrate/025_add_tag_support.rb

@@ -17,6 +17,8 @@ class AddTagSupport < ActiveRecord::Migration
   end
 
   def self.down
+    remove_index :taggings, [:tag_id, :taggable_id, :taggable_type]
+    remove_index :tags, :name
     drop_table :taggings
     drop_table :tags
   end