Make Q&A the default spam countermeasure, and add an optional install step to configure it.

As discussed in this topic: http://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=42745
Q&A has become the only anti-spam system that really works, because it is the only one that requires people to think about the input to it. As such, it is a good idea to make Q&A the default anti-spam system, and to provide the ability to set it up during installation. While this does add an extra step, we will make it optional, with a prominent Skip button. Nils also suggested adding a Skip button at the top of the Server Settings page, since that page is also optional.