Add register / guest_login spam detection
captchas are considered bad practice. The following links provide
alterantive ideas:
http://docs.typo3.org/typo3cms/extensions/wt_spamshield/Spamshield/Introduction/WhatDoesItDo/Index.html
http://playground.ebiene.de/antispam-bee-wordpress-plugin/#function
http://blog.doctima.de/2012/11/bitte-nicht-bessere-alternativen-zu-captchas/
http://www.usertesting.com/blog/2014/04/09/think-your-site-needs-captcha-try-these-user-friendly-alternatives/
http://www.sitepoint.com/captcha-alternatives/
http://www.1ngo.de/web/captcha-spam.html
http://www.wob11.de/captchashowall.html
(from redmine: issue id 1846, created on 2014-09-15 by gmueller)