Enable SPARQL endpoints as data sources

[k00ni]

• [x] Backend can be a Virtuoso
  • authentification is not neccessary
• [x] Map RDF data to an internal object model
• [x] Reactivate SPARQL queries to gather data
• [x] Testing: rewrite tests, code coverage
• [x] Caching: use localStorage or sessionStorage to store objects (infos about space restrictions https://stackoverflow.com/questions/2989284/what-is-the-max-size-of-localstorage-values)
• [x] Lazy preloading of data

RDF data dump can be found here: https://github.com/AKSW/transform-bvl-pages-to-csv-file/blob/master/places.n3

Timetable:

• [x] General feedback in calendar week 16 to @k00ni
• [x] Calendar week 17 provide demo for tests