Scala test fw

First attempt to use the inhouse Scala testing framework with one test for checking the picture overlay functionality. Keep in mind that I am a beginner Scala dev so code will likely look very "Javaish"
Not sure I understand exactly what's going on here ... 

If you want to make sure that a method is implemented, you can just do

```scala
def url: String
```

(There's no need to do `= null`, and you shouldn't.)