Added a property on both fixture and team responses called
is_placeholder. This property indicates if the resource is used to display dummy data. The false of this property will always be a
boolean value. An example where this becomes useful: Imagine you want to load the complete schedule of the European Championship and display this in a nice visual. You would need the complete schedule including knockout games which haven't yet been determined when the season starts. This property tells you that there the data is not final or is used for placeholder purposes. The id of a fixture will remain the same when real data is merged to it.