Add read file in bytes using fileb://

Fixes https://github.com/aws/aws-cli/issues/814, https://github.com/aws/aws-cli/issues/815

If you want to read the file as binary, simply prepend file name with ``fileb://`` instead of ``file://``

cc @jamesls @danielgtaylor 
I would be good with changing that as it would mean less code to maintain. I only originally had it in there to avoid the least amount of change in behavior. Yet again *if* anything changes in the debug output, it should be minimal because we would only be using ``repr()`` instead of ``str()``