Remove Purifier's transparent-extension-enabling file.  Since Purifier gets loaded first, nothing can extend it anyway.
Move Purifier class into the main directory now that the transparent-extension-enabling file is gone.

--HG--
rename : modules/purifier/classes/Purifier/Purifier.php => modules/purifier/classes/Purifier.php
Big rewrite of Purifier class (needs testing still).
- move config from separate file to static variable in class (cascading config
files aren't too useful when 0 other modules are loaded).
- add ability to add new config group and HTMLPurifier instance if desired.
- add Purifier::purify_array() to clean GPC superglobals.
Add superglobal XSS cleaning code to bootstrap.  Revise Purifier to use Kohana::find_file().