Changing the way we authenticate to the API and adding some tests on each resources

What's Changed

• Adding tests by @ngiraud in #4
• Removing the Client::makeWithClientCredentials() method
• New way to authenticate through the Spotify class :
  • Spotify::client('access-token')
  • Spotify::basic('client-id', 'client-secret')

Full Changelog: 1.0.3...1.0.4