Anonymous voting

It does. Give anonymous users the permission to "vote on content". They will be able to vote only once for a given content with an IP address for a certain duration of time. That 'certain duration of time' is set through the configuration page of the Voting API module.

You are looking at the wrong branch.

What you're looking at is deprecated.

Here is the theming function:

  if (!$logged_in || user_access('vote on content')) {
    $output .= '<div class="'. $message_class . '">';
    if (!$logged_in && !user_access('vote on content')) {
      $output .= '<small>'. l(t('Log in<br />to vote'), 'user', array('html' => TRUE)) .'</small>';
    }
    else if ($voted) { // User already voted.
      $output .= check_plain(variable_get('plus1_you_voted', t('You voted')));
    }
    else if (user_access('vote on content')) {
      // User is eligible to vote.
      // The class name provided by Drupal.settings.plus1.link_class what we will search for in our jQuery later.
      $output .= '<div class="plus1-vote">'. l(t('Vote'), 'plus1/vote/'. $node->nid, array('attributes' => array('class' => $link_class))) .'</div>';
    }
    $output .= '</div>';
  }

Anonymous voting is indeed not supported in the first branch.