Number Guessing Game - Update 4

Added feature to allow users to choose difficulty.
Added comments to make code more readable.
Validates user's guesses according to difficulty chosen.
Integrated use of user's name to provide a more user-centered
experience.
Added a personal logo generated from http://patorjk.com/
Increased minimum number of moves to compensate for increased
difficulty.

To Do:
- Allow user to navigate back to Intro screen from the Help menu.
- Randomly generate encouragement for the user.