SFTP/Stream: use $scheme instead of 'sftp'

This makes it easier for someone to change the scheme (which can be
done by changng the stream_wrapper_register call at the bottom
of the file)
SFTP/Stream: add explanation as to why nlist() is used here
SFTP/Stream: fix capatilization
Merge pull request #352 from terrafrost/sftp-stream-changes

SFTP Stream changes

* terrafrost/sftp-stream-changes:
  SFTP/Stream: fix capatilization
  SFTP/Stream: add explanation as to why nlist() is used here
  SFTP/Stream: use $scheme instead of 'sftp'