Fix autoloading in Composer.
Fix autoloading in Composer.

Per https://getcomposer.org/doc/04-schema.md#psr-0

ezyang/htmlpurifier,bartbutler/htmlpurifier,r-kovalenko/htmlpurifier,Masterjoa/HTMLPurifier-standalone,stefanotorresi/htmlpurifier,bartbutler/htmlpurifier,Janson-Leung/htmlpurifier,Janson-Leung/htmlpurifier,r-kovalenko/htmlpurifier,Janson-Leung/htmlpurifier,ezyang/htmlpurifier,ezyang/htmlpurifier,Janson-Leung/htmlpurifier,stefanotorresi/htmlpurifier,stefanotorresi/htmlpurifier,Masterjoa/HTMLPurifier-standalone,r-kovalenko/htmlpurifier,bartbutler/htmlpurifier,bartbutler/htmlpurifier,stefanotorresi/htmlpurifier,r-kovalenko/htmlpurifier,ezyang/htmlpurifier