Fixed/added SimpleDB, DynamoDB, EMR, and EC2 methods to return Streams on native methods that chunk results.

Fixed DynamoDB batchGet for situations where the batch request size exceeds the DynamoDB limit. Now, batchGet returns a Stream[Item]. (The design of the solution is the same as the S3 listObjects design.) This fixes #33.

Also fixed SimpleDB.domains. 
Remove these imports and use `aws.model.TagDescription` instead. The reason for that is to distinguish between AWS SDK's Java types and AWScala's Scala types.