add SolrResponse#documents to translate solr response documents to a blacklight::solr::document model
Extract SolrDocument references from controllers and models and define
the solr document class to use in the blacklight configuration.

- add blacklight_config.solr_document_model configuration (defaults to
  SolrDocument)
- make Bookmarks polymorphic and refactor Bookmarks to use objects
  instead of passing IDs around
Merge pull request #855 from projectblacklight/solr-document-class

Extract explicit SolrDocument class references to make the model class configurable