Implement VALUES syntax and operation

[niklas88]

This is the original VALUES implementation extracted from #258.

Note: This needs the fix to the galloping join from #269 to function as it easily triggers the broken case for example when using a single value as in the following example query (which only gave one result instead of >7000):

PREFIX wd: <http://www.wikidata.org/entity/>
PREFIX wdt: <http://www.wikidata.org/prop/direct/>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX schema: <http://schema.org/>
PREFIX wikibase: <http://wikiba.se/ontology#>
SELECT ?city ?cityLabel ?citytype WHERE {
  VALUES ?citytype { wd:Q515}
  ?city wdt:P31 ?citytype .
  ?city rdfs:label ?cityLabel .
  FILTER (lang(?cityLabel) = "en") .
}

[niklas88]

Tested and reviewed this LGTM