Allow setting the timezone for sprintf in Logstash event class

A regular problem that arises for users is that Logstash uses UTC time **everywhere**. Although this is suitable and usually highly desirable for timestamps, there are other places where using UTC doesn't make sense and leads to unexpected results for end-users.

One particular example is with the elasticsearch output plugin as discussed here:

https://github.com/logstash-plugins/logstash-output-elasticsearch/issues/165

Allowing for example, an option to change the default timezone for everything but timestamps would result in a better experience for the end-user, many of whom expect the (in reference to the issue above) indexes to be rotated on local time rather than UTC and better integration with other tools/services on the server, like cron (and curator called through cron), which usually operate in local time.

The following branch seems to have the beginnings of a fix for this:

https://github.com/elastic/logstash/compare/elastic:master...talevy:sprintf-with-tz