Uploading listens (Last.fm Scrobbler 2.0 compatibility)

[aramperes]

Use-Case Description

A compatibility endpoint for Last.fm Scrobbler 2.0. This will help integrating with software & apps that are already using Last.fm.

Depends on #19. Will land in v2 of the API.

Semantics

Should be compatible with the schema from https://www.last.fm/api/scrobbling