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.
Hey @stigi, isn't it enough by checking the NSOperation key paths (isReady, isExecuting, isFinished and isCancelled)?