Add plugin for radius authentication

Plugin for RADIUS authentication (similar to the ftp plugin, no user listing, only authentication. Master/Slave Mode recommended!).

PHP Radius support is needed. If you get an php segfault its die to an php-radius bug fixed with newer versions (https://bugs.php.net/bug.php?id=60885).

I dont know if my logoutCallback does the right thing, but it has worked at my test setup.