CC-3636: Media Folder: dir path in cc_music_dir should be in consistent format

- DateHelper.php is moved from model to common folder
- created OsPath.php file which contains normpath()
- normpath() is used instead of realpath()

airtime_mvc/public/index.php

@@ -18,6 +18,8 @@ set_include_path(implode(PATH_SEPARATOR, array(
     realpath(APPLICATION_PATH . '/../library')
 )));
 
+set_include_path(APPLICATION_PATH . '/common' . PATH_SEPARATOR . get_include_path());
+
 //Propel classes.
 set_include_path(APPLICATION_PATH . '/models' . PATH_SEPARATOR . get_include_path());