Add a propriety for the user’s name

Adds the public NSString* propriety named userName.
Implement the changeGreeting: method

Get the text from the text field and set the userName propriety (model object) to its value. Create a new NSString from the model object and test if its length is 0. If it is then put “World” as default NSString. Finally construct the greeting and set the label text to its value.
Hide keyboard when the user taps Done

Implement the textFieldShouldReturn: method of the UITextFieldDelegate protocol, and call resignFirstResponder on the UITextField* in order to hide the keyboard when the user taps Done.