code: function _numberToKey(index, totalChar, offset) {
      var sKeys = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=_!?()*',
        aArr = [],
        currentChar = index;
      totalChar = totalChar || sKeys.length;
      offset = offset || 0;
      while (currentChar >= totalChar) {
        aArr.push(sKeys.charCodeAt((currentChar % totalChar) + offset));
        currentChar = Math.floor(currentChar / totalChar - 1);
      }
      aArr.push(sKeys.charCodeAt(currentChar + offset));
      return aArr.reverse();
    }
docs: Convert a number to their ascii code/s.
  @param index
  @param totalChar
  @param offset
  @returns {Array}
  @private