2.11: Podcasts, shows and episodes

Added

• Support for shows/podcasts and episodes
• Added CONTRIBUTING.md

Changed

• Client retry logic has changed as it now uses urllib3's Retry in conjunction with requests Session
• The session is customizable as it allows for:
  • status_forcelist
  • retries
  • status_retries
  • backoff_factor
• Spin up a local webserver to auto-fill authentication URL
• Use session in SpotifyAuthBase
• Logging used instead of print statements

Fixed

• Close session when Spotipy object is unloaded
• Propagate refresh token error