Replace Number with Scientific

Scientific numbers are floating-point numbers with arbitrary precision
that are fast to parse and can be efficiently converted to/from
Fractional numbers like Doubles.
Change encoding of Rational to use full precision
Add cabal.sandbox.config to .gitignore
NaNs and infinite RealFloats are encoded to Null
Refactor the FromJSON instance of Fixed
Merge pull request #156 from basvandijk/scientific

Change Number to Scientific