Adopt logstash "schema" version 1

This patch set adopts version 1 of the Logstash "schema", or convention (in use since Logstash 1.2.0)

This mostly required a simple gem upgrade for logstash-event, some test patching, and only minor tweaks to the actual code itself. Feel free to ping for any desired discussion.

The schema/convention itself is discussed [here](https://logstash.jira.com/browse/LOGSTASH-675).