Rename to `nerves_hub_device`

[fhunleth]

At the risk of introducing a painful project name change, I'm having lots of trouble verbally communicating with people about this library. "NervesHub", the server, and nerves_hub, this package, sound the same. To limit confusion, I've been calling this "NervesHub device" or "nerves underscore hub".

I want to start this issue so that the decision process on the name, if any, is logged and I can point people to it when they ask. I'm not in any rush to commit to a new name and I'm not set on nerves_hub_device if there are other better ones. If we're going to switch, I'd like it to be before 1.0.