Fixed bugs within addition and subtraction wrappers.
Now they admit and correctly operate with both positive and negative
numbers.

Fixed bug within isGreaterThan.
Before the cases of both positive and both negative were treated the
same, but negative numbers need inverse logic.

Tested exhaustively all cases for the Addition, Subtraction and School
multiplication.

Set up a representative case that exposes a bug in Karatsuba
multiplication (filling with zeroes is incorrectly done, check toString
method to see a correct application).