Test doubles

The folder timezones contains a simple project that gets timezone information from the World Time API.

The class TimeZoneService depends on WorldTimeApiHttpClient that consults the API, we need to test TimeZoneService but WorldTimeApiHttpClient relies on the external API.

1. Create an interface for WorldTimeApiHttpClient, name it just WorldTimeApiClient.
2. Use the new interface to create a fake class.
3. Use the fake class to create a unit test testing TimeZoneService.