_civicrm_registerClassLoader() should store a relative path

https://issues.civicrm.org/jira/browse/CRM-13737 introduced a Drupal variable 'civicrm_class_loader' to cache the physical path of the class loader.

This has caused problems when restoring site backups to with a different physical path structure. See http://forum.civicrm.org/index.php/topic,31366.msg134439.html and https://issues.civicrm.org/jira/browse/CRM-14186. In my case, the presence of this variable resulted in a WSOD site and where drush cannot even bootstrap. I had to manually clear the 'civicrm_class_loader' from the variables table, plus clear the 'variables' row of the cache table.

These problems are avoided by modifying _civicrm_registerClassLoader() to store a relative path rather than the physical path.