Adds WikibaseEntityResourceNode

Allows to keep mapping between string and Wikidata entities and to improve user interface
Adds description to WikibaseEntityResourceNode

Useful for simple output of the result (we have a label, a description and a link)
Adds specification of wikibase-entity resource node in README
Merge pull request #37 from ProjetPP/wikibaseEntityResourceNode

Adds WikibaseEntityResourceNode