Support for more "float-likes", Bignum, Non-String values.

- Replaced check for Fixnum by check for Integer to handle Bignum in the same way as Fixnum
- Replaced separate checks for Float and BigDecimal by a more generic check for Numeric and to_f
  to include support for Rational and maybe more exotic floating point numbers while not building
  any dependency on BigDecimal
- Added call to to_s for other values which is a no-op for String
- Added specs to check correct handling of Float, BigDecimal and Rational amounts
Merge pull request #8 from TheConstructor/Rational

Support for more "float-likes", Bignum, Non-String values.