code: function randomString(_string_length, _mode) {
      
      var string_length = _string_length;
      if(string_length < 3)
      	string_length = 2;
      
      var chars = chars_all;
      
      if(_mode === 'lower_case') chars = chars_lower;
      else if(_mode === 'upper_case') chars = chars_upper;
      else if(_mode === 'nums_oly') chars = chars_nums;
      else if(_mode === 'especials') chars = chars_all + chars_spec;
      
      var str = '';
      for (var i=0; i<string_length; i++) {
          var rnum = Math.floor(Math.random() * chars.length);
          str += chars.substring(rnum,rnum+1);
      }
      
      return str;
  }
docs: Generates a random string of the provided length, and including only
  the required characters.
  
  @param {int} _string_length
  @param {String} _mode
  @api public