Collective.captcha

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- Add a captcha field and widget for zope.formlib.

- Reduced the characters used to generate the captcha to uppercase letters and
numbers only; user testing showed that even humans had trouble solving the
captchas.

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- Delete the captcha cookie from the response if one is present to
prevent premature expiration. This would happen when creating a
new captcha after verification (because verification failed or
the form would have to be redisplayed for other reasons). Thanks to
David Glick for pointing this out.

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- Initial release