Implementing the prerequisites for background fetch

In order to implement background fetch, you will need to take the following three steps:

1. Add the background fetch capability to your app.
2. Ask iOS to wake your app up.
3. Implement application(_:performFetchWithCompletionHandler:) in AppDelegate.

We'll implement step 1 and 2 right now; step 3 will be implemented separately because this step will involve writing the code to fetch and update the movies using a helper struct.