Restructure Renameuser extension

I've restructured the Renameuser extension so that there's one
php file for every class which can all be autoloaded. That's a
much more sane and more modern way of organizing classes and it
makes external usage much easier.

Usage example: https://gerrit.wikimedia.org/r/39171

On top of that I've created a README and a hooks.txt file.

Change-Id: Iee10b10934fa3cda608ed2230db16e75b30afcc6
Merge "Restructure Renameuser extension"