Allow to configure the path to the custom error pages

This PR adds a simple way to tell Hipache where to find the static error pages. It is useful when one wants to customize those pages:

``` json
  {
      "server": {
          "workers": 10,
          "maxSockets": 100,
          "deadBackendTTL": 30,
          "staticDir": "/etc/hipache/static"
      },
      "http": {
          "port": 80,
          "bind": [ "0.0.0.0" ]
      },
      "https": {
          "port": 443,
          "bind": [ "0.0.0.0" ],
          "key": "/etc/ssl/ssl.key",
          "cert": "/etc/ssl/ssl.crt"
      },
      "driver": "redis://redis"
  }
```

Not sure about the `null` default value.

Then again, test suite is broken, so I can't really add test right now, which is... annoying. Anyway, you can test this feature using my [`Hipache` image](https://registry.hub.docker.com/u/willdurand/hipache/).
`var staticDir = config.server.staticDir || [ rootDir, 'static' ].join('/');`

Also, put that out of the staticPath function itself.

What do you think?