Exposing notifications when network connections are started/stopped

In some scenarios it's necessary to expose the information when a network connections starts or ends. For example when using an own networking activity indicator.

Therefore I've added notifications to `LRImageOperation` whenever the connection is started, finishes or fails that can be forwarded.
I think it's common practice to use notifications for this matter. It's much more convenient to have a central place to update the network activity indicator than to observe every operation created all over the app.

Also UIKit categories like `-lr_setImageWithURL:` don't expose the operation and therefore don't allow to KVO them.