Drilldown changes

Added a few changes in code & tests for drill down feature.
I somehow end up adding an entry <Service Include="{82A7F48D-3B50-4B1E-B82E-3ADA8210C358}" /> to the integration test project inadvertently. Is there something to be careful to avoid it? It's there in this changeset too, so please undo it.

Another question, cloudant search doc on drilldown (https://docs.cloudant.com/api/search.html) says that this can be specified multiple times, whereas query doesn't seem to support it (its a query param so only a single entry of name:value can go in as the value type seems to be a 2 valued JSON array). Any ideas? For now, the code appends a single drilldown at a time.