Initial commit

Hi,
I have enjoyed a lot coding this test code. It has been very funny.
I have implemented a UICollectionView as a man view (Master VIew) to show the list of the most popular pictures from 500px. In a first time, I thought to do it with a UITableView, But with the requirements of the test I decided finally with a UICollectionView.  I have used core data: The first time you run the application the list is downloaded from 500px, but the nexts times it reads the list from internal BBDD. This is easy to change or maybe we can insert a button at top of the view to reload the data from 500px.
I have used constraints and autolayout and I have implemented a simple animation to show the the details. 
I have tried to use ios design patterns like: MVC, singleton, decorator, observer, etc.

If you have any question feel free to ask me.

Thank you so much

Kind regards.
¿Porque weak?
¿Esto podria conllevar algun tipo de problema?