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 
Could we just use %r always?  I think we if we remove the quotes from the "%s" it should look almost the same.

```
'Unpacked value of %r for param %s: %r'
```