Ensure msgid gets serialized correctly

Right now this is being built as a base for a client that will be used
specifically with turntable.fm's websocket api. In their api, the id
field *must* be named `msgid`.

I tried to turn `Message` into a generic trait, and that was a nightmare
of a process, so I'm just hard coding this for now.

We also need to flatten the data field, so this adds that functionality.