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
I found some more bugs in upgrading to 1.3.0, so I reverted to 1.2.6 in this PR.