DATAMONGO-960 - Allow to pass options to the Aggregation Pipeline.

We introduced AggregationOptions abstraction to conveniently construct option objects that can be handed to an Aggregation via the new Aggregation.withOptions(...) factory method. For more details, see the Builder class' JavaDoc.

Note that we exposed the "rawResults" in AggregationResults and put a null guard in MongoTemplate aggregate in order to support the "explain" option.

Original pull request: #195.