introduce root property

this lets you do this:

```
{
  "name": "mydata",
  "root": "http://max.com/mydata",
  "resources": [
     {
        "name": "data.csv",
        "path": "/data.csv"
     }
  ]
}
```

also, ideally you could just do `["/data.csv"]` for resources, but that should be another PR

the benefit here is that you can easily switch out where data files should come from without rewriting the entire resources array