Issue #178: Contextual Action Bar

@codeguru42 It looks great. Your work is very systematic and organized. I now understand how to make small specific changes and commit it regularly. It makes it very easy to understand. 
I have one doubt : I see that onResume of BaseballCardsList is removed. So have you decided to remove the functionality to retain the status of selected cards.

Also, I am not sure if it is my merge issue. But the app crashes when I select a few cards or all cards, exit the app by pressing home button, then open the app again. Can you please check if this scenario works for you.