Retire Echo Nest plugin in favor of new Spotify API

The Echo Nest API is shutting down. From an email I got today:

> 1. March 29th, 2016 - As of March 29, 2016, we are announcing three new APIs (details below) and The Echo Nest API will no longer be issuing new API keys
> 2. May 31, 2016 - As of May 31, 2016 The Echo Nest platform will no longer serve requests and developers will need to move over to the Spotify API.

The news could be worse; they've simultaneously [added equivalent features to the Spotify API](https://developer.spotify.com/news-stories/2016/03/29/audio-features-recommendations-user-taste/). But we need to migrate, and it seems that the new endpoint requires authentication with a per-user Spotify account.

Start by locating the Echo Nest plugin and the code that handles its API requests. Read the linked Spotify API announcement and determine the authentication and feature-migration requirements; the work is done when Echo Nest requests are replaced and the supported equivalent functionality works with Spotify authentication.

Written by the indexing model from the issue text.