Add DBALQueryInterface

Further todos relevant to this code:

* https://github.com/wmde/WikibaseDatabase/issues/30 QueryResult inconsistency
* https://github.com/wmde/WikibaseDatabase/issues/59 DBALQueryInterface::select should use $options parameter
As said, this is completely unrelated to this pull request (you are not even introducing the "integration" directory here, it was there before). Please do this in a separate patch.