add support for thumbnails in search results

Fixes #580

Add support for:

```
  configure_blacklight do |config|
    config.index.thumbnail_method = :xyz
  end
```

and

```
  configure_blacklight do |config|
    config.index.thumbnail_field = :some_field_in_the_document
  end
```

Given that, Blacklight will render a thumbnail (by default, right aligned, under the bookmark button) as a link to the document page. If no thumbnail is available (missing data in solr, the thumbnail method returned nil), the thumbnail block is not displayed at all.