simple schema.org support

e.g.

```
    config.add_index_field 'author_display', :label => 'Author:', :itemprop => 'author' 
```

```
class SolrDocument
  ...
  def itemtype
     first("my_schema_org_type_field") || "http://schema.org/CreativeWork"
  end
end
```

@awead? @jkeck? @jronallo? 

This doesn't support any of the advanced schema.org features (see http://schema.org/docs/gs.html#advanced). I think we could tackle 3a. Not sure about the others. I'd propose we merge this without support for those features, but happy to hear otherwise.