RecaptchaPlugin: fix for missing captcha on iPhone/Android.

RecaptchaPlugin: fix for missing captcha on iPhone/Android.

MobileProfile serves pages out to iPhone and Android as application/xhtml+xml, which doesn't work with the default we we were loading recaptcha (as it used document.write). Switched to filling out a <div> from the AJAX API, which doesn't use document.write in the XHTML context.

Tested that view & submission works ok in following browsers:
Mobile: iPhone 3.1, Android 2.1, iPad 3.2 (this last doesn't trigger mobile theme tweaks)
Ubuntu 10.04: Firefox 3.6.3, Chrome 6
Mac 10.6: Safari 5/OS X 10.6.4
Windows 7: IE 8, Opera 10.56