Added annotations for course variables

lib/common/messages.dart

@@ -1,13 +1,30 @@
 library titama.messages;
 
+import 'package:rpc/rpc.dart';
+
 class Course {
+  @ApiProperty(required: true)
   String title;
+
+  @ApiProperty(required: true)
   String time;
+
+  @ApiProperty(required: true)
   String day;
+
+  @ApiProperty(defaultValue: "")
   String kind;
+
+  @ApiProperty(defaultValue: "")
   String place;
+
+  @ApiProperty(defaultValue: "")
   String prof;
+
+  @ApiProperty(defaultValue: "")
   String turnin;
+
+  @ApiProperty()
   int id;
 
   Course();