Improve list bucket invalid and bad auth tests

Explicitly test invalid access keys and valid access keys but invalid
secret keys.  Also disable anonymous list bucket test for AWS.
radosgw uses this for connectivity testing but AWS does not allow it:

http://ceph.com/docs/master/radosgw/config/#verify-the-runtime
http://docs.aws.amazon.com/AmazonS3/latest/API/RESTServiceGET.html
Merge pull request #24 from andrewgaul/list-bucket

Improve list bucket invalid and bad auth tests