Backwards-compatible way of calling Finance both as an instance object and namespace-class

[igorsantos07]

Superseding #23 with a backwards-compatible way of calling Finance both as an instance object and namespace-class

[oneforwonder]

I agree this would be a significant improvement to the API. This library is simply a collection of mathematical functions that stand in isolation. The Finance class contains no data and performs no initialization. It is not needed.