Added some stuff and fixed a bug.
- Various API and non-API changes (0.y.z).
- #14 Bad negative versus infinite comparison
- #13 DataInt/isNormal
- #12 BinaryInteger/isZero and DataInt/isZero
- #11 Finite<T> and Natural<T> trusted inputs
- #10 FiniteInteger(s)
- #9 GCD and XGCD algorithms
