function _arrayBufferToString(buf, callback) {
    var bb = new Blob([new Uint8Array(buf)]);
    var f = new FileReader();
    f.onload = function(e) {
      callback(e.target.result);
    };
    f.readAsText(bb);
  }
Converts an array buffer to a string

@private
@param {ArrayBuffer} buf The buffer to convert
@param {Function} callback The function to call when conversion is complete