Update GetQueueAttributes signature and add test for issue 20

I also ran a separate test against SQS and verified the logic worked.
I agree with @Mistobaan. I am not really a fan of the way the DeleteMessageBatch has been implemented and I think it can benefit from some proper refactoring going forward. 

1) I am not convinced by the way we check for errors here. Unless I am missing something, I don't see anywhere in the spec where it says that the SenderFault flag is always true. It might as well be that there is a problem on the other end. 
2) Ultimately it should be client app's responsibility to deal with the errors in the batch.  Simply logging an error count and returning makes this impossible. 

I believe the Response object needs to match exactly the AWS API in this case and unmarshal both the successful changes and any failures in the appropriate objects. Any client can handle the response after that.