Add basic client structure. Add test.
Modify the Client struct to easily be able to change the API url.

This change should only be used for testing purposes.
Add test helpers. Add first test for Client.Artist.
Add Artist and Image struct. Add a custom JsonURL struct. Add Artist Unmarshall test.

The JsonURL struct has been added to allow JSON Marshal/Unmarshal of
url.URL without having to write a full custom Marshaller/Unmarshaller
for Artist and Image. If you want to access the url.URL, just do
field.URL, assuming field is a JsonURL.
Add Artist Marshal test.