Fixed the base (was wrong, our base must be the highest digit + 1, more
concretely 999.999.999 + 1 = 1.000.000.000 ).

Fixed the addition and substraction methods, now they comply with the
base. Also added the last carry, which wasn't being added to the data
vector.

Fixed the toString method to display correctly elements of the data
vector that do not have 9 digits (fill with zeroes), excepting the most
significative element, which is left untouched.

Implemented and tested the School multiplication method. NOTE: Casts to
long of each of the multiplication operands was necessary in order for
it not to flood.