Update geshi.php, add CAPTCHA

This includes a couple of things:

* Adds a simple math CAPTCHA to the contact form
* Updates geshi.php to 1.0.8.11.

The GeSHi update is to fix deprecation warnings in recent versions of PHP:

    Deprecated: preg_replace(): The /e modifier is deprecated, use preg_replace_callback instead in [...path...]/site/plugins/highlight/geshi.php on line 2139