ContactInterface should extend EntityChangedInterface

First of all, thank you for the work on the Examples modules for D8. Thanks to its code and the documentation the learning curve for D8 has been made easier for me.

Now to the issue: the Contact entity has the getChangedTime() method which it is supposed to have inherited from the ContactInterface interface. However, ContactInterface does not extend EntityChangedInterface which provides the method.

So if my observation is correct it should just be a quick fix.