Support for s3n:// in S3Filesystem.ls()

I've added support for the `s3n://` schema when calling the `ls` method on `S3Filesystem`. This allows us to use a composite filesystem with `S3Filesystem` in front of `HadoopFilesystem` to gain extra performance (boto is much faster than hadoop for s3).

This fix transparently passes though `s3n://` to `s3://` for boto and then back to `s3n://` for the list of uris returned.