gistfile1.php

<?php

function taleo_libraries_info() {
  return array(
    'Taleo' => array(
      'name' => 'Taleo PHP Library',
      'xautoload' => 'taleo_xautoload',
    ),
  );
}
function taleo_xautoload($api) {
  // $api already knows the library path, so all paths are relative to that.
  // If you still want absolute paths, you need to path a third argument FALSE.
  // See http://drupalcode.org/project/xautoload.git/blob/refs/heads/7.x-2.x:/lib/InjectedAPI/hookXautoload.php
  // $api->namespaceRoot('Guzzle', $path . '/vendor/guzzle/guzzle/src', FALSE);
  $api->namespaceRoot('Guzzle', 'vendor/guzzle/guzzle/src');
  $api->namespaceRoot('Monolog\Handler\StreamHandler', 'vendor/monolog/monolog/src');
  $api->namespaceRoot('Monolog\Logger', 'vendor/monolog/monolog/src');
  $api->namespaceRoot('Taleo\Main\Taleo', 'src');
}

Duh, of course those relative paths need to be without the slash.