[3.1.0] More PHP4->PHP5 conversions, notably reference removal of most methods that return objects

- Removed HTMLPurifier_Error
- Documentation updates
- Removed more copy() methods in favor of clone
- HTMLPurifier::getInstance() to HTMLPurifier::instance()
- Fix InterchangeBuilder to use HTMLPURIFIER_PREFIX

git-svn-id: http://htmlpurifier.org/svnroot/htmlpurifier/trunk@1689 48356398-32a2-884e-a903-53898d9a118a

library/HTMLPurifier/ConfigDef.php

@@ -3,7 +3,7 @@
 /**
  * Base class for configuration entity
  */
-class HTMLPurifier_ConfigDef {
+abstract class HTMLPurifier_ConfigDef {
     public $class = false;
 }