Make ExchangeRate dated

[RemyDuijkeren]

See the comments of #8 for the start issue.

ExchangeRate is/can't be dated at the moment. It would be nice to add the dates where the ExchangeRate is valid between.

Possible implementations could be:

// override of constructor
var fx = new ExchangeRate("EUR", "USD", 1.14m, dateFrom, dateTo)
var fx = new ExchangeRate("EUR", "USD", 1.14m, new Range<DateTime>(dateFrom, dateTo))
var fx = new ExchangeRate("EUR", "USD", 1.14m, new Period(dateFrom, dateTo))

// or be external
var fx = new ExchangeRate("EUR", "USD", 1.14m)
var datedFx = new Range<DateTime>(dateFrom, dateTo, fx);
var datedFx = new Period(dateFrom, dateTo, fx);

[JayT82]

In case of ExchangeRate, this issues gives a solution in the example code.
There might be some DataServices that give Exchange rates with a start date and validity duration.