How to add rel="author" attribute to Drupal 7?
The rel=author property is now supported by the Drupal 7 Metatags module dev version.

After installing the metatags module (http://drupal.org/project/metatag), go to admin/config/search/metatags, and edit the front-page or one of the node pages. Under 'Advanced' you will see the author and publisher fields where you can input your google+ page URL. On the following screenshot, I've marked the places you need to change the configuration in pink.

Metatags frontpage configuration page: