Feature/custom observers

```objc
self.observer = [AQSNotificationObserver observerWithBlock:^(NSDictionary *userInfo, id object) {
	// Called when "FooNotification" is posted.
} name:@"FooNotification"];
```

Or with object,

```objc
self.observer = [AQSNotificationObserver observerWithBlock:^(NSDictionary *userInfo, id object) {
	// Called when "FooNotification" with object: `someObject` is posted.
} name:@"FooNotification" object:someObject];
```