Convert GND-Mappings provided by DNB

[nichtich]

https://data.dnb.de/opendata/ currently contains 166.056 Mappings from GND to other vocabularies. We only have 43.396 of these as of 2016 (Project MACS).

Would also solve #40 (but only updates every six month).

[nichtich]

Its in directory gnd but

• missing Thesoz, LCSH, and RAMEAU mappings so far
• date of update not included (required for regular updates)
• no null-mappings included (hard to do in core SKOS, possible in JSKOS)

[stefandesu]

Added DDC and LCSH. I switched to parsing N-Triples with JavaScript for conversion, but so far I have copied the script instead of trying to generalize so that it could be used for other vocabularies as well.