[2.0] Refactor KVO in categories

Smashing `observeValueForKeyPath:ofObject:change:context:` in categories is a very bad idea. :scream_cat:

Here’s my progress so far to eliminate this behavior.

- [x] Remove KVO in UIActivityIndicatorView+AFNetworking  
I used notifications instead of KVO.
- [ ] Properly handle this case

```objc
[activityIndicatorView setAnimatingWithStateOfTask:taskA];
[activityIndicatorView setAnimatingWithStateOfTask:taskB];
```
- [ ] Make sure everything is OK if the UIActivityIndicatorView is deallocated while the task is still running.
- [x] Remove KVO in UIProgressView+AFNetworking
- [ ] Properly handle this case

```objc
[progressView setProgressWithUploadProgressOfTask:taskA animated:YES];
[progressView setProgressWithUploadProgressOfTask:taskB animated:YES];
```
- [ ] Make sure everything is OK if the UIProgressView is deallocated while the task is still running.
- [ ] Write unit tests. I’d love to write some unit tests but it seems that there’s a lot to be done for the tests to work on the 2.0 branch.