Fixed issue with serving static content

The `static` config property supports serving static content from the Venus http server.

For example, let's say you have a config file in `~/project/.venus/config`.

In this config file, you have:

```js
{
  static: {
    mystuff: '../static'
  }
}
```

When Venus runs, the files under `~/project/static/` will be available through the URL `http://hostname:port/temp/static/mystuff`.

This was broken, but this pull request restores the functionality and adds test coverage for this feature, to protect against future regressions.