Add support for describeStream API variant with exclusiveStartShardId

[tzulitai]

Recently found out that Kinesalite doesn't seem to support one of the describeStream() variants:
http://docs.aws.amazon.com/AWSJavaSDK/latest/javadoc/com/amazonaws/services/kinesis/AmazonKinesisClient.html#describeStream-java.lang.String-java.lang.String-

The API response is supposed to only include shardIds larger than the exclusiveStartShardId , but it doesn't seem to be working as expected with Kinesalite.