Implementation for path_alias_xt

Mostly an fyi: I posted a patch for the path_alias_xt module to use Drupal Override Function for its runkit usage instead of directly. See #962194: Use drupal_override_function's hook_function_overrides for runkit.
Nothing has to be done on this module's side for that to work (other than #961814: Error "canary mismatch on efree" from runkit_function_rename which is a general holdup), but it might be useful to track modules that use this one.