Support for Logstash v1 event schema (and improved timestamp handling)

With version 1.2.0 Logstash introduced a new event schema.  Salt is sending the old-style events with the logstash log handler module.  While it is possible to deal with old-style events on newer Logstash versions, its better long-term to start sending new-style events.  These patches add that support (defaulting to the old-style events).  Also, improvements are made to timestamp handling - there's no need for doing anything fancy with pytz and timezones, logstash is perfectly happy with UTC timestamps.  Also, these patches reduce the precision of timestamps to milliseconds - the library that logstash uses to deal with time (JodaTime) can't handle anything more, and certain versions will choke if given anything more.