ENH: More permissive S3 reading
When calling `get_bucket`, boto will by default try to establish that the S3 bucket exists by listing all of the keys that exist in it. This behavior is controlled by the "validate" keyword, which defaults to True. If your access key doesn't have permission to read everything in a bucket (even if you do have permission to read the file you're trying to access), this generates an uninformative exception. 

This PR sets "validate=False". This means that boto will trust you that the bucket exists, and not try to check immediately. If the bucket actually doesn't exist, the `get_contents_as_string` call a couple of lines later will generate the exception "S3ResponseError: S3ResponseError: 404 Not Found".
