2.13: Implicit Grant Flow

Added

• Added SpotifyImplicitGrant as an auth manager option. It provides
  user authentication without a client secret but sacrifices the ability
  to refresh the token without user input. (However, read the class
  docstring for security advisory.)
• Added built-in verification of the state query parameter
• Added two new attributes: error and error_description to SpotifyOauthError exception class to show
  authorization/authentication web api errors details.
• Added SpotifyStateError subclass of SpotifyOauthError
• Allow extending SpotifyClientCredentials and SpotifyOAuth
• Added the market parameter to album_tracks