Implement getQueueAttributes

First commit is to make the `awsjs.SQSClient` creation a little more dry. I have it isolated so please let me know in case this is not ok and I'll send you a new PR.

Other commits implement Issue #7

syntax:
```javascript
  sqs.getQueueAttributes(/*string*/ queue [, /*Array*/ attrs], /*function (err, result) */ fn );
```
where result is formatted as: `{attr-1: value-1, attr-n: value-n}`