2 show acwa

Added message methods to Problem model because
 (1) to make it possible to change message dynamically (e.g. change messages according to small or large, display special message if both are solved, etc.)
 (2) I felt Problem model is responsible for messages (at first I thought of Contest model, but thinking (1), I concluded that AC/WA messages are per-problem things).

Please merge if it seems sane.

[Future Work]
Need to improve UI (though it is not a problem only for AC/WA messages).
I want to talk about UI someday.