close defunctzombie/zuul#103

It was impossible to run a testing support server from another directory
because of the following code in `lib/user-server.js`

```
var dir = process.cwd();
```

Therefore, I needed to add support to the following format with dir option:

```
server:
  ./test/support/server.js:
    dir: ../anotherapp
```

https://github.com/defunctzombie/zuul/issues/103
should follow existing code style