Extra Credit 1: "backspace" button

[petervanhoef]

Implement a “backspace” button for the user to touch if they hit the wrong digit button. This is not intended to be “undo,” so if the user hits the wrong operation button, he or she is out of luck! It is up to you to decide how to handle the case where the user backspaces away the entire number they are in the middle of typing. You will probably find the Strings and Characters section of the Swift Reference Guide to be helpful here.