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();
    }
Convert a number to their ascii code/s.
@param index
@param totalChar
@param offset
@returns {Array}
@private