handle itemref attributes

[jronallo]

The itemref attribute needs to be made part of the parsing algorithm. The use of itemref in the wild might be minimal, but microdata.py will currently give incorrect results when parsing a document that uses itemref.

You can use this example document extracted from the spec:
https://github.com/jronallo/microdata/blob/master/test/data/example_itemref.html
http://www.whatwg.org/specs/web-apps/current-work/multipage/microdata.html#the-basic-syntax

The results should look like this:

{
  "items": [
    {
      "properties": {
        "name": [
          "Amanda"
        ],
        "band": [
          {
            "properties": {
              "name": [
                "Jazz Band"
              ],
              "size": [
                "12"
              ]
            }
          }
        ]
      }
    }
  ]
}