Create initial version of BestStatementsFinder

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`