code: function(from, to) {
        return new Promise((resolve, reject) => {
          util.mkParentPath(to, function() {
            var rs = fs.createReadStream(from, { flags: "r", encoding: "binary" });
            var ws = fs.createWriteStream(to, { flags: "w", encoding: "binary" });
            rs.on("end", function() {
              resolve();
            });
            rs.on("error", reject);
            ws.on("error", reject);
            rs.pipe(ws);
          });
        });
      }
docs: Copies a file
  @param from {String} path to copy from
  @param to {String} path to copy to
  @param cb(err) {Function}
  @async