Add podcasts support

[sophiedeziel]

Adding partial support for Shows and Episodes that were added to Spotify's API.

This PR does not add support for podcasts under the /me/... URLs, as I wish to get feedback on this first part before I open a new Pull Request implementing the user-specific features around podcasts.

Summary of the changes

• The Show class is created
• The Episode class is created
• Links between the two classes were made through the Show#episodes and Episode#show methods
• Corresponding tests were added
• Lots of new cassettes for VCR

Note: I had to cheat the cassettes a little bit, as the client_token needs to be one of a user. So I recorded the cassettes with authentication that I did not want to commit and then I searched and replaced my tokens in each file with the one already in use for the tests. There are some improvements I could submit as another PR to provide a better method of authenticating for tests that does not require to commit tokens and secrets.

[guilhermesad]

This is great, thank you!