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
What will `e.(string)` do with non-string values (e.g. numbers)? (Probably time to write some tests, eh.)