Use axios instead of requests package (deprecated)

[eebmagic]

Switch from using requests library to axios for post and get calls (The requests library has been deprecated).

The axios package has been listed as an alternative by the requests developers.

Also addresses some deprecation warnings for creating Buffers (See nodejs page here).

Resolves this GitHub issue:

• https://github.com/spotify/web-api-auth-examples/issues/49