Added Addition Method

The user can now enter addition and will be prompted to enter two
numbers. On the completion of entering the numbers the result will be
displayed. If the user enters an operation other than addition they are
notified that the operation is invalid.
Added Loop

The program will now execute on a loop until the user prompts the
program to exit.
The program now uses switch statements instead of if, else if, else etc.
The program now uses BufferedReader instead of Scanner. I was having
problems with the while loop where after executing the addition
operation it would run a second time without any input from the user.
Switching to the BufferedReader fixed this.