Add support for BigQuery query plan explanation

Based on the recent [BigQuery update](http://googlecloudplatform.blogspot.com/2015/12/BigQuery-cost-controls-now-let-you-set-a-daily-maximum-for-query-costs.html) we should add support for query plan explanation. Plan explanation is contained in the [queryPlan field](https://cloud.google.com/bigquery/docs/reference/v2/jobs#statistics.query.queryPlan) inside query job's statistics.
