API, part I

Introduce a "v1-alpha" HTTP API: 
- Both the server and client side has been prototyped out, so it should be smooth sailing from here
- fleetctl has a new `--experimental-api` flag that can be used to interact with the new API
- `GET /machines` is the only implemented resource
- A document reflecting what is currently implemented has been added with a big warning about the experimental nature of the API
- This is already safe to ship as it requires a user to take additional steps to socket-activate fleet
same comment as earlier re: using composite literals