Add annotation code.

Need to figure out how we are getting the annotation data, and how we want to present it to users for use. Is it an additional field where they enter their own endpoint to query, and that endpoint needs to know to respond with unix time and a message?

@juliusv @grobie 

requested in #115/#187
depends on https://github.com/soundcloud/slog/pull/26
It's fine. Just remember for the release notes to also include this fix :)