First attempt to convert Fraction to using BigDecimal and BigInteger.  All ant tests pass.  Haven't yet checked the code for things that need tidying up.  Also, Term evaluation, and the downstream operations involving Functions, are still being done using doubles rather than BigDecimals, because Java doesn't provding a pow(...) or log(...) function for BigDecimal.
Trivial tidying up.
Merge pull request #59 from stepthom/issue55

Issue55