Makes the Ghost application more express middleware friendly. 

refs #827

- Moves ./index to use Ghost in a similar manner to how someone uses
Ghost as an npm module.
- Allows Ghost to be cleanly mounted on another express application
on any arbitrary endpoint, all you need to customize is the mount path.

I built this on top of my #3866 pull request, hence the second commit.


In `./index`, instead of starting ghost by doing:

```
instance.start(server);
```

You can swap it to:

```
server.listen('2368')
```

Voila.