Expose GetLatestRates() publicly as a List<ExchangeRate>

[Arithmomaniac]

[rmorrin]

This is something I'd initially considered as a way of exposing all data provided by the API.

I think it makes sense to pass a value for the base parameter, so something like GetLatestRates("USD") to get all rates from USD to other currencies, will have a think about the best approach.

If you have any suggestions yourself (or want to submit a PR!) I'd be more than happy to take a look. 👍