*
@method shift(<n>)
@returns Array
@short Shifts each character in the string <n> places in the character map.
@example

'a'.shift(1)  -> 'b'
'ク'.shift(1) -> 'グ'

*
function(n) {
      var result = '';
      n = n || 0;
      this.codes(function(c) {
        result += chr(c + n);
      });
      return result;
    }