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
Yep, this file is auto-generated by contrib/schema-generator. I was hoping to make the minimal changes to this file to make it work for us so maintaining it is simpler:

1. Change `package fleet` to `package schema`
2. Rewrite the `code.google.com/p/google-api-go-client/googleapi` import

To be clear, I have already made these two changes, which explains why the file-level doc string still references the `fleet` package.