Classify answers by round in answerlist.php with GET arg. (fixes #65)

* If answerlist.php receives an argument `rid`, it prints two lists of
  available answers, one of those in round `rid` and one of those not.
  If no argument is given, proceed as before.
* This feature is only available by manually setting a GET argument,
  since it is apparently only intended for people who Know What
  They're Doing (TM).