Multi machine rehaul

Fixes #25:

- Adds ShoveInstance model for tracking individual instances of Shove, and a management command for receiving heartbeats from shove instances with info about them, such as their hostname and what projects they support.
- Updates the interface to allow for sending commands to a user-selectable list of shove instances that have reported that they support the project in question. Also includes a SentCommand model for tracking individual command runs across multiple shove instances.
- ScheduledCommands now include a list of hostnames that their repeated commands will be sent to. This way you can ensure your repeating command runs on the same host even if the shove instances goes down and comes back up with a different queue name.

mozilla/shove#15 is the shove side of this change. I haven't updated the docs yet as I want to test this out a bit more heavily before bothering to document it, in case we run into any major issues.