Added COMMANDO_PLUGINS and TFTP_HOST to settings
Adjustments to allow for sending kwargs to xmlrpc methods.
Added 'force' option to add_handler method
Various plugin tweaks.

- Moved xmlrpc_config_device() into config_device module.
- Removed 'self' from args in xmlrpc_* methods.
- Changed config_device logging to use twisted.python.log
Removed xmlrpc_config_device() from xmlrpc/server.py.
Various fixes to plugables.

- Changed xmlrpc_execute_commands to accept only two args: args, kwargs.
- Changed xmlrpc_execute_commands to send *args,**kwargs to CommandoApplication.
- Fixed typo where 'dummy' xmlrpc method was not returning anything (made for fun troubleshooting).
added show_clock and show_version to the COMMANDO_PLUGINS in global_settings
Removed class_name from required plugin attributes.
Fixed (kw)args notation in plugins.