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.
Done in [here](https://github.com/luisrecuenco/LRImageManager/commit/9a49a393285a93082a51c6f6a7baace8049a609b) via LRImageManager. LRImageCache doesn't have anything to do with requesting network images, so we shouldn't declare those string constants there. Also, LRImageOperation is sort of private, it's something only the manager uses (the presenter as well), so it makes sense to listen for this notifications from the manager's perspective, not the notification. Makes sense?