ZF-2859: Add CAPTCHA element to Zend_Form


The ability to easily add CAPTCHAs to a form would greatly enhance Zend_Form's offering. Stas has already prototyped some code for inclusion with the library, and we should evaluate for the next mini release.


Resetting 'fix version priority' and 'fix version' to be re-evaluated for next release.

Scheduling for next minor release.

Functionality and tests complete; documentation remains.

Merged to trunk, as well as to 1.6 release branch.

Updating for the 1.6.0 release.