register_user plugins are changed into component; some bugfixes; update POD

 - register_user plugins are changed into component
    - Plugin/AutoRegisterUser.pm -> Component/AutoRegisterUser.pm
    - Plugin/CustomRegisterUser.pm -> Component/CustomRegisterUser.pm
    - remove load_components('Autocall::Autoload') from Uc::IrcGateway.pm
 - bugfix: Common::to_json overrides common JSON instance settings when it calls with options
 - bugfix: exception_cb can't die if $self->condvar is set

 - update POD
   - Common.pm: SYNOPSIS and INTERFACE
   - add =cut at tail of each files