Fix s3 output plugin

This rewrite takes care of a few of the TODOs in the original plugin, namely getting it using the awsconfig mixin that the other AWS related plugins use, and syncs to S3 on teardown.

Most of this is aped from the file output so hopefully should be pretty solid, but let me know if anything's amiss as it's my first time writing a plugin for logstash and really using Ruby at all.  Also - this includes https://github.com/logstash/logstash/pull/924 as it was required to get AWS connectivity working properly.