Added new database singleton class.
Added new database singleton class.

Instead of directly calling following statement:
  $pdo = new PDO($connStrPDO);
Let's do like this:
  $pdo = DB::getConnection();

By doing this, we can avoid creating multiple
connections from one request.

utrad-ical/circus_cs_web_ui,utrad-ical/circus_cs_web_ui