PREREQUISITES: Make sure you check HTML Purifier and make sure that you
have fulfilled all of its requirements before running this.  Specifically,
you'll need the PHP extension ctype (in almost all PHP distributions),
and it's nice to have dom and iconv.

* Place the htmlpurifier folder in your drupal modules directory.

* Download HTML Purifier from http://htmlpurifier.org/ You will need
  4.0.0 or later.

* There are two locations you can install the HTML Purifier library.

  1. Module directory installation. This means installing the library
     folder under the module directory, so that the file
     sites/<site>/modules/htmlpurifier/library/HTMLPurifier.auto.php
     exists.  The easiest way to do this is to extract the entire
     htmlpurifier-x.y.z folder, and then copy the
     htmlpurifier-x.y.z/library folder to your module.  This method is
     convenient and simple, but does not permit HTML Purifier to be shared
     with other modules and can make upgrading the Drupal module a little
     complicated.

  2. The preferred way is making use of the libraries API,
     http://drupal.org/project/libraries. This makes the library
     available to all sites or to a specific site in a multisite
     Drupal setup. You'll need to download the libraries API module
     and enable it before enabling the htmlpurifier module so that in
     the install phase it can find the library.

     Extract the htmlpurifier-x.y.z archive to
     sites/all/libraries/htmlpurifier or to
     sites/<site>/libraries/htmlpurifier for a specific site in a
     multisite Drupal setup.  (Note that this is different from the
     module directory installation; you need the *entire* folder, not
     just the library folder).

     The final setup should be, when making the library and module
     available to all sites:

     sites/all/libraries/htmlpurifier/library
         HTMLPurifier
         HTMLPurifier.autoload.php
         HTMLPurifier.auto.php
         HTMLPurifier.func.php
         HTMLPurifier.includes.php
         HTMLPurifier.kses.php
         HTMLPurifier.path.php
         HTMLPurifier.php
         HTMLPurifier.safe-includes.php

     Now you can safely upgrade your htmlpurifier module without
     having to re-deploy the HTML Purifier library.

* Go to Administer > Modules and enable this module

* You can now create a new text format or add the HTML Purifier to an 
  existing text format. It is recommended that you place HTML Purifier as 
  the last filter in the text format. Reorder the filters if necessary.

WARNING: Due to HTML Purifier's caching mechanism, dynamic filters MUST NOT
be placed before HTML Purifier.