Fix to expand refs before creating root editor

[ianmetcalf]

I ran into an issue when using $ref in the root schema, where the schema in the file referenced doesn't get expanded. Here is an example that reproduces this:

var editor = new JSONEditor(element, {
  ajax: true,
  schema: {
    $ref: '/schema/a.json',
  }
});

{
  "id": "/schema/a.json",
  "oneOf": [
    {
      "$ref": "/schema/b.json"
    },
    {
      "$ref": "/schema/c.json"
    }
  ]
}

I fixed this be expanding the refs in the root schema before instantiating the root editor.

Also, I had to use node v0.10 in order to run grunt. You may want to make a note of that in the README or upgrade the grunt jshint package to support newer versions of node.