implement post change report

app/logical/google_big_query/post_version.rb

@@ -0,0 +1,15 @@
+module GoogleBigQuery
+  class PostVersion < Base
+    def find_removed(tag, limit = 1_000)
+      tag = escape(tag)
+      limit = limit.to_i
+      query("select id, post_id, updated_at, updater_id, updater_ip_addr, tags, added_tags, removed_tags, parent_id, rating, source from [#{data_set}.post_versions] where regexp_match(removed_tags, \"(?:^| )#{tag}(?:$| )\") order by updated_at desc limit #{limit}")
+    end
+
+    def find_added(tag, limit = 1_000)
+      tag = escape(tag)
+      limit = limit.to_i
+      query("select id, post_id, updated_at, updater_id, updater_ip_addr, tags, added_tags, removed_tags, parent_id, rating, source from [#{data_set}.post_versions] where regexp_match(added_tags, \"(?:^| )#{tag}(?:$| )\") order by updated_at desc limit #{limit}")
+    end
+  end
+end