Add an interface module to the WikidataQuery API.

Methods to generate the queries programmatically from either integer IDs or
Pywikibot page classes like ItemPage are provided, along with basic caching
of results. Tests included.

Change-Id: Id1dd2c48c65b9bfb877ec10ad1b8ea69aa00a39c
Merge "Add an interface module to the WikidataQuery API."