This commit makes clear that the list methods for the Competition/SoccerSeason resource only return data for the current season or for any targeted season. Fetching the complete list is not possible.
- Make the "Client" struct construction-friendly so it can be easily initialized with "new(footballdata.Client)".
- Rewrite example_test.go to show the respective new usage. Make it clear here that the SetToken call is optional.
This would fix#2.