Add problems 1-4.
Added Set, Map, Counter, and Factors classes.  Solved problem 5.
Solved problem 6.
Solved problem 7.
Solved problem 8.
Solved problem 9.
Solved problem 10.
Used the op.* functions in earlier problems.
Solved problem 11.
Made Factors toString to a math expression.
Solved problem 12. Added Factors#countDivisors().
Solved problem 13. Added Bignum class.
Fixed a few errors.