In Go rewrite:  Move service to its own folder, add ENV.

Some work I did over the weekend and this morning:

- Move `service.go` to its own folder (eventually this should be done with at least `command.go` too when it gets bigger).  This has the cool side effect of allowing easy imports from other Go programs, for instance I'm using it for a little side project I'm hacking on.
- Add `Env` option to `Create`.  Now environment variables work (with slice of `KEY=VALUE` format)!
- Move `TimedEvent` code to its own file.
- Add remaining parameters for unmarshalling from `fig.yml` to `Service` struct
- Change help text ever so slightly to be more like existing fig
Heh.  Not particularly pretty:

```
web:
    build: .
    environment:
     FOO: 3000
     QUUX: SPAM
```

will give:

```
        "Env": [
            "FOO=%!s(int=3000)",
            "QUUX=SPAM",
            "HOME=/",
            "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
        ],
```

What is expected `fig` behavior when using `3000` instead of `"3000"`?  Environment variables are basically all strings, but it should handle gracefully when this is not the case in `fig.yml`.