Big update of 6.x-2.x branch (keyword: hook_elements)

I'm currently busy with a rather big refactoring/rewrite of the 6.x-2.x branch.
At the core of this rewrite is the usage of the drupal hook hook_elements() to define the CAPTCHA stuff, instead of doing it in hook_form_alter(), which was getting a bit messy.

It makes the code much cleaner and enables more flexibility/control concerning the adding and placement of CAPTCHA widgets to forms.
E.g.: the CAPTCHA placement can be done more powerful, other modules can add CAPTCHA widgets without using the CAPTCHA admin UI and it could also enable a CAPTCHA CCK field.

The code is still under development, but I think it makes sense to post the current state. In attachment is a patch agains CVS HEAD
If you're familiar to git, you can also follow the development more closely at http://github.com/soxofaan/drupal-captcha-dev/tree/hook_elements