Added a regression test for SQS's delete behavior & updated the docs.

This is intended to address https://github.com/boto/boto/issues/1300. However, it doesn't pass for me yet. It's having to do extra cleanup from the ``SQSConnectionTest.test_1_basic`` method, which leaves queues hanging around long after they're no longer needed.

I'll do the work to finish it, but any pointers would be much appreciated if I'm doing something obviously stupid.

Additionally, running the entire integration suite (for SQS) is proving to be rather slow. Is there a better command than ``python test.py -v -t sqs`` to be running to test just the ``SQSConnectionTest`` class?