adds in functionality for direct download and dct references parsing

An implementation of the dct:references proposed schema using current `selected.json` values.

Stanford records are the only ones which currently have direct download as a reference. This pattern will also be able to parse the metadata uri's in a similar fashion and potentially eliminates the need to include `solr_wms` field in solr schema. We should be able to add in logic to parse references for preferred tile server (wms, wtms, iiif) and preferred metadata.

Closes #131 


Yes, I changed it to http://schema.org/downloadUrl in my PR yesterday. The idea with the test cases for dct:references is that you can load them into your index rather than selected.json or maybe afterward. It'll be a while before Kim and I can generate the OGP network index for selected.json.