[add] NSCopying protocol support to TNCheckBoxData class and subclasses

[k06a]

I would like to use it this way:

TNCircularCheckBoxData *adminCan = [[TNCircularCheckBoxData alloc] init];
adminCan.identifier = @"adminCan";
adminCan.labelText = @"Only admin can invite members";
adminCan.labelColor = [UIColor colorWithWhite:74/255. alpha:1.0];
adminCan.labelWidth = 220;
adminCan.checked = YES;
adminCan.borderColor = [UIColor colorWithWhite:216/255. alpha:1.0];
adminCan.circleColor = [UIColor colorWithRed:0 green:200/255. blue:83/255. alpha:1.0];
adminCan.borderRadius = 30;
adminCan.circleRadius = 24;

TNCircularCheckBoxData *everyoneCan = [adminCan copy];
everyoneCan.identifier = @"everyoneCan";
everyoneCan.labelText = @"Everyone can invite members";
everyoneCan.checked = NO;