DATAMONGO-753 - Add support for nested field references in aggregation operations.

Aggregation pipelines now correctly handle nested field references in aggregation operations. We introduced FieldsExposingAggregationOperation to mark AggregationOperations that change the set of exposed fields available for processing by later AggregationOperations. Extracted context state out of AggregationOperation to ExposedFieldsAggregationContext for better separation of concerns. Modified toDbObject(…) in Aggregation to only replace the aggregation context when the current AggregationOperation is a FieldExposingAggregationOperation.

Original pull request: #74.