#192 top hits aggregation support

I tried,

I feel that elasticsearch org.elasticsearch.search.aggregations.metrics.tophits.TopHitsBuilder https://github.com/elasticsearch/elasticsearch/blob/master/src/main/java/org/elasticsearch/search/aggregations/metrics/tophits/TopHitsBuilder.java#L35 should extend MetricsAggregationBuilder[B] or AggregationBuilder[B] and not just extend AbstractAggregationBuilder to easily fit into elastic4s. Currently I implemented just few methods from this builder, but AFAIS not all builders are fully implemented in elastic4s, e.g. CardinalityBuilder is not reflected.

Anyway - let me know if this PR needs some improvement to merge it