Update to 0.15

Upstream changes:
0.15  9 December 2012

    - Lots of internal changes to Ei, li, Zeta, and R functions:
      - Native Zeta and R have slightly more accurate results.
      - For bignums, use Math::MPFR if possible.  MUCH faster.
        Also allows extended precision while still being fast.
      - Better accuracy for standard bignums.
      - All four functions do:
          - XS if native input.
          - MPFR to whatever accuracy is desired, if Math::MPFR installed.
          - BigFloat versions if no MPFR and BigFloat input.
          - standard version if no MPFR and not a BigFloat.

    - Add tests for primorial, jordan_totient, and divisor_sum.

    - Revamp of the random_prime internals.  Also fixes some issues with
      random n-bit and maurer primes.

    - The random prime and primorial functions now will return a Math::BigInt
      object if the result is greater than the native size.  This includes
      loading up the Math::BigInt library if necessary.

(wen)