add twilio notification plugin

Signed-off-by: Dave Simons <dave@inuits.eu>
notifyxmpp: fix parameter checks, even though they are duplicating puppet core functionality

Signed-off-by: Dave Simons <dave@inuits.eu>
notifytwilio: fix dependency on icinga::config class

Signed-off-by: Dave Simons <dave@inuits.eu>