Change update API and add observers

[TomasKypta]

Add observers for updates and global observers for validation failures. Change update method to be asynchronous with observer. Add method for checking if update is necessary (without invoking the update).

[petrdvorak]

... or we should have first talked about the observer callback naming - the correct one is onYouCanContinueFriendlyDeveloper(). This would emphasize that from our consumer developer perspective, the network activity and updates are irrelevant and should be hidden...

[petrdvorak]

On the other hand, if we just add UpdateType to the onUpdateFinished method, it will work and in a way, the API will give our developer more control. We can also then implement class alongside these lines:

public class DefaultUpdateObserver implements UpdateObserver {

   public void onUpdateStarted(UpdateType type) {
        if (type == UpdateType.SILENT || type == UpdateType.NO_UPDATE) {
            runTask(); // run the task now...
        }
    }

    public void onUpdateFinished(UpdateType type, UpdateResult result) {
        if (result == UpdateResult.OK) {
            if (UpdateType == UpdateType.DIRECT) {
                runTask(); // run the task later
            }
        }
    }

   public abstract void runTask();

}

[TomasKypta]

OK, I can we can extend onUpdateFinished. It's not necessary, you can easily work around it, but it simplifies things for the devs. And the default is some a nice to have.