2.8: Auto-refresh token

Added

• Support for playlist_cover_image
• Support after and before parameter in current_user_recently_played
• CI for unit tests
• Automatic token refresh
• auth_manager and oauth_manager optional parameters added to Spotify's init.
• Optional username parameter to be passed to SpotifyOAuth, to infer a cache_path automatically
• Optional as_dict parameter to control SpotifyOAuth's get_access_token output type. However, this is going to be deprecated in the future, and the method will always return a token string
• Optional show_dialog parameter to be passed to SpotifyOAuth

Changed

• Both SpotifyClientCredentials and SpotifyOAuth inherit from a common SpotifyAuthBase which handles common parameters and logics.