CDEvent should maintain strong references to its properties.

Unretained property is source of headaches:

```objc
- (void)foo {
    self.events = [[CDEvents alloc] initWithURLs:URLs
                                           block:^(CDEvents *watcher, CDEvent *event) {
                                               self.lastEvent = event;
                                           }];
}

- (void)bar {
    // Remember the URL property is unsafe_unretained.
    NSLog(@"%@", self.lastEvent.URL); // BOOM!
}
```