Uses currency rates API to perform conversion and comparison tasks

Has the follwing functions:

1. getLatestRates- returns a JSON string that is a response to a latest rates query. The Json string will have the attributes: rates, base and date (yyyy-mm-dd).
2. changeBase- Converts currency from one base to another for a given date.
3. printAscending- Prints rates of all currencies in increasing order
4. extremeFridays- Given a start date and an end date, tells on which friday was currency the strongest and on which was it the weakest.