Add an extension for logging MediaWiki events to the system logger

Add an extension for logging MediaWiki events to the system logger
(syslog). Although intended to be useful, it's more of a testbed for
event hooks than anything else. For a first pass, started logging user
login and user logout events. Had to add hooks-running code to the
login and logout modules, but that's kind of the point. Documented the
three events added in hooks.doc.