Provide a CaptchaWrapper

[cmb69]

The old CAPTCHA "interface" is still useful, but could benefit from some encapsulation. Just wrote a CaptchWrapper for Advanceform_XH that might be a reasonable addition to Plib_XH. Should double-check with Twocents_XH.

[cmb69]

Should double-check with Twocents_XH.

Relevant differences:

1. no explicit ::include() method
2. passes an $admin parameter
3. only calls the functions if they are actually callable (not sure why I didn't use function_exists() there)

(2) is doubtful; it's a minor convenience, but seems to be too special. (1) and (3) may or may not be good ideas.