Add image method to Forecast model

It's possible to retrieve the icon representing the forecast from yr.no's
servers and this method makes it easier to do so.
Remove means to get Forecast image with only ID

The class should only be concerned with integrating with the Yr API and such
a feature (albeit helpful in certain situations) is generally unnecessary.
Add tests for Forecast::image
Merge pull request #1 from nodesagency/feature/forecast-image

Add Forecast::image method