captcha 6.x-2.0-rc1

Changes since DRUPAL-6--2-0-BETA5:

• #463002: dropping the preprocess op in hook_captcha()
• hardcoded CAPTCHA placement on comment_form: the 'Submit' button can be omitted on first submission, which confuses the automatic placement detection.
• fixed problem with CAPTCHA description still showing up after comment preview with correct CAPTCHA response and added test for this
• #467618: made CAPTCHA element placement more foolproof, e.g. when there are no buttons on a form
• added forum_node_form to the default form_ids on install
• fixed problem with node previews (CAPTCHA session ID was not reused, so no multipage form persistence)
• captcha_set_form_id_setting() now also accepts an object with module and type attributes
• #472144 by hctom and soxofaan: PHP error on CAPTCHA admin form submit when no CAPTCHA points defined