Support for serving linked data

These commits implement support for serving PeriodO as linked data, including a [VoID](http://www.w3.org/TR/void/) description of the dataset. The following is an outline of the resource classes:

*   `/`
  * if `Accept: text/turtle`: `void:DatasetDescription` document
  * if `Accept: text/html`: PeriodO client
* `/.well-known/void`
  * `void:DatasetDescription` document
* `/dataset`
  * `void:Dataset` (abstract subject of VoID statements); `303 See Other` to `/dataset/`
* `/dataset/` *or* `/dataset.json` *or* `/dataset.jsonld`
  * entire dataset as JSON-LD
* `/dataset/#periodCollections`
  * `rdf:Bag` (abstract root resources for easier access to period collections, each of which is a `rdfs:member` of it)
* `/8mbcl`
  * `skos:ConceptScheme` (abstract period collection)
  * if `Accept: text/html`: `303 See Other` to `/#8mbcl`
  * if `Accept: application/json`: `303 See Other` to `/8mbcl.json`
  * if `Accept: application/ld+json`: `303 See Other` to `/8mbcl.jsonld`
* `/8mbcl.json` *or* `/8mbcl.jsonld`
  * JSON-LD representation of a period collection
* `/8mbcl/pr72`
  * `skos:Concept` (abstract period definition)
  * if `Accept: text/html`: `303 See Other` to `/#8mbcl/pr72`
  * if `Accept: application/json`: `303 See Other` to `/8mbcl/pr72.json`
  * if `Accept: application/ld+json`: `303 See Other` to `/8mbcl/pr72.jsonld`
* `/8mbcl/pr72.json` *or* `/8mbcl/pr72.jsonld`
  * JSON-LD representation of a period definition