[WIP] Create initial version of BestStatementFinder

This is some initial idea to fix #190. It still awaits an answer to https://github.com/wmde/WikibaseDataModel/issues/190#issuecomment-56076159 because the usage of `ByPropertyIdArray` is not that nice in this context.

### TODO
* Finish tests
* Find an alternative for `ByPropertIdArray`
This is not how the original `ByPropertyIdArray` was intended to be used. You'd construct the thing where you wanted to map the data. You'd not demand the data in this form.