Consistent Field Names

Please create common field types (AWS Region, Event Source, etc) that the event struct's can embed. There seems to be inconsistency between similar event types, such as [SQS](https://github.com/aws/aws-lambda-go/blob/master/events/sqs.go#L19) or [Kinesis](https://github.com/aws/aws-lambda-go/blob/master/events/kinesis.go#L10).