Introducing errors with private attribute.

Exercise 11 (b).
Add exercise 11 (c).

Public method Number.printNumber ().
Add exercise 11 (d).

Added constructor for Number.
Add exercise 11 (e).

Added public method Number.absolute ().
Add exercise 11 (f).

Added public method Number.sum (int n).
Add exercise 11 (h).

Added over written methods Number.add () for float, and double.
Add exercise 11 (i).

Added public method Number.toDouble ().
Add exercise (j).

Added public method Number.getX ().