fixes 3 bugs, and implements 1 suggestion (factor, Laurent polynomials):

- bug #98. factoring Laurent polynomial now works properly.
- related bug. numerator / denominator now work on Laurent polynomials and provide sensible answers.
- isPrime has been rewritten in a clean, factor-order-agnostic way. fixes such bugs as "3*x is not prime in QQ[x]".
- #39. factor now chooses the sign of each factor so that its leading monomial has positive coefficient (if basering is ZZ or QQ; behavior undefined for other baserings)