Fix queuing for parallel requests

This release fixes bug #54, which would cause concurrent code such as:

ftp.ls(path1, function showFile(err, res) {
  if (err) return console.log(err);
  console.log(res[0].name);
});

ftp.ls(path2, function showFile(err, res) {
  if (err) return console.log(err);
  console.log(res[0].name);
});

Would always show the same result, because items in the queue haven't been shifted.