Add setMessage() to update notification message as will

Sampling use case:

```
self.permanentNotification = [CSNotificationView
notificationViewWithParentViewController:self.navigationController
                                                     tintColor:[UIColor colorWithRed:0.000 green:0.6 blue:1.000 alpha:1]
                                                         image:nil message:[NSString stringWithFormat:@"Uploading file %@...", self.fileName]];
[self.permanentNotification setShowingActivity:YES];
[self.permanentNotification setVisible:YES animated:YES completion:^{}];
```

Somewhere you want to update the percentage:

```
[self.permanentNotification setMessage:[NSString stringWithFormat:@"Uploading file %@ (%1.0f%%)", name, percent*100]];
```