Add support for serialization to JSON

lib/common/messages.dart

@@ -30,4 +30,17 @@ class Course {
   Course();
 
   String toString() => title.isEmpty ? 'notitle' : title;
+
+  Map toJson() {
+    Map map = new Map();
+    map["title"] = title;
+    map["time"] = time;
+    map["day"] = day;
+    map["kind"] = kind;
+    map["place"] = place;
+    map["prof"] = prof;
+    map["turnin"] = turnin;
+    map["id"] = id;
+    return map;
+  }
 }