code: function _numberToKey(index, totalChar, offset) {
      var aArr = [],
        currentChar = index;
      totalChar = totalChar || 26;
      offset = offset || 65;
      while (currentChar >= totalChar) {
        aArr.push((currentChar % totalChar) + offset);
        currentChar = Math.floor(currentChar / totalChar - 1);
      }
      aArr.push(currentChar + offset);
      return aArr.reverse();
    }
docs: Convert a number to their ascii code/s.
  @param index
  @param totalChar
  @param offset
  @returns {Array}
  @private