code: function computeHash(filename, algo, callback) {
      var crypto = require('crypto');
      var fs = require('fs');
  
      var chksum = crypto.createHash(algo);
  
      var s = fs.ReadStream(filename);
      s.on('error', function (err) {
          //no file, hash will be zero
          callback(0, err);
      });
      s.on('data', function (d) {
          chksum.update(d);
      });
  
      s.on('end', function () {
          var d = chksum.digest('hex');
          //util.debug(d + '  ' + filename);
          callback(d);
      });
  }
docs: Create a new hash of given file name