Add messages count to API response for logged in user details

Allows external editors to alert the user to the fact that they
have unread messages on the OSM site.

app/views/user/api_read.builder

@@ -41,6 +41,11 @@ xml.osm("version" => API_VERSION, "generator" => GENERATOR) do
           @this_user.languages.split(",") { |lang| xml.tag! "lang", lang }
         end
       end
+      xml.tag! "messages" do
+        xml.tag! "received", :count => @this_user.messages.size,
+                             :unread => @this_user.new_messages.size
+        xml.tag! "sent", :count => @this_user.sent_messages.size
+      end
     end
   end
 end