Modified XSS Cleaning routine to be more performance friendly and compatible with PHP 5.2's new PCRE backtrack and recursion limits.

- replaced link and image tag javascript sanitization preg_replace()'s with callback functions to avoid excessive backtracks on strings with many links / image tags.
diff --git a/user_guide/changelog.html b/user_guide/changelog.html
index 58bf1b6..09800ed 100644
--- a/user_guide/changelog.html
+++ b/user_guide/changelog.html
@@ -70,6 +70,7 @@
     <li>Fixed a bug in the <a href="./libraries/zip.html">Zip library</a>, providing PC Zip file compatibility with Mac OS X</li>

     <li>APPVER has been deprecated and replaced with CI_VERSION for clarity. </li>

     <li>Added array to string into the profiler </li>

+	<li>Modified XSS Cleaning routine to be more performance friendly and compatible with PHP 5.2's new PCRE backtrack and recursion limits.</li>

     <li>Fixed a bug in router that was ignoring the scaffolding route for optimization </li>

     <li>Fixed doc typos. </li>

 </ul>