Added private method to calculate a player's score
Edited and defined methods to get/set attempts list
Finished call method
I.e. it now updates attempts list for the quiz and returns the score to
the calling method
implemented submitAnswers method.
Most of work done by callable, so just generally copied the statements
of the other methods the submit callables and return results from
futures.
Made update of attempts list synchronised
Rewrote interface and class.
TempQuizInfo now stores the list of questions to be answered only. It is
easy enough to iterate through this list when required. It also stores
an error message that is set if the quiz cannot be fetched. This will
happen if a bad quizID is passed or if the quiz has closed in the time
between the player viewing all available quizzes and choosing to play
it.
Added constructor that takes error message
Updated call method to reflect 3 options:
1. quiz is found and questions loaded
2. quiz id doesn't exist
3. quiz is closed

I changed the associated tests as well.
Added reminder to insert synch block that increments # of active games
Added reminder to decrement # of active games