Support "one-click" uploading of Infobox data to Wikidata

[siznax]

It would be helpful to everyone if we could submit Infobox data that this tool finds in various Wikimedia projects to Wikidata when it is helpful. We should offer a simple method to make this easy, for example wptools.wikibase.upload(). Here's what we need to accomplish this:

1. Determine if this is possible via the Wikidata API
   • wbcreateclaim?
   • wbsetreference?
   • wbcheckconstraints?
2. Offer "references" data with infobox data (#114)
3. Add a "safe" upload method:
   • only adds statements to existing Wikidata entities
   • satisfies "Add only verifiable information" requirement
   • does not clobber existing statements
   • validates uploader input
   • responds to error codes

Comments, suggestions welcome!