Added pixiv: metatag and Pixiv source indexes

Adds a pixiv: metatag to quickly find posts by Pixiv ID.  Adds
source:pixiv/* searches to allow left-anchored source searches for Pixiv
sources.  Added relevant indexes to make these queries efficient.

db/migrate/20130302214500_add_index_pixiv_on_posts.rb

@@ -0,0 +1,15 @@
+class AddIndexPixivOnPosts < ActiveRecord::Migration
+  def up
+    execute "set statement_timeout = 0"
+    execute "CREATE INDEX index_posts_on_pixiv_suffix ON posts USING btree
+             ((substring(source, 'pixiv.net/img.*/([^/]*/[^/]*)$')) text_pattern_ops);"
+    execute "CREATE INDEX index_posts_on_pixiv_id ON posts USING btree
+             ((substring(source, 'pixiv.net/img.*/([0-9]+)[^/]*$')::integer));"
+  end
+
+  def down
+    execute "set statement_timeout = 0"
+    execute "DROP INDEX index_posts_on_pixiv_suffix;"
+    execute "DROP INDEX index_posts_on_pixiv_id;"
+  end
+end