Add google_cloud_storage output plugin and google-api-client gem

This plugin buffers files locally and upload them to Google Cloud Storage after being rolled (based on time, size, or both). It's been tested locally using a GCS account with multiple different configuration files (when not running Logstash as a jar file). Let me know if you have any comments.

For logstash release, this gem will require that fix in the JRuby monkey patches to work in a jar (please refer to the discussions regarding Faraday and JRuby a few weeks back).