Adding composite aggregation, including an example helper

Fixes elastic#902

Author: honzakral

elasticsearch_dsl/aggs.py

@@ -209,6 +209,13 @@ class Sampler(Bucket):
 class DiversifiedSampler(Bucket):
     name = 'diversified_sampler'
 
+class Composite(Bucket):
+    name = 'composite'
+    _param_defs = {
+        'sources': {'type': 'agg', 'hash': True},
+        'aggs': {'type': 'agg', 'hash': True},
+    }
+
 # metric aggregations
 class TopHits(Agg):
     name = 'top_hits'

examples/README.rst

@@ -20,3 +20,11 @@ For simplicity we use a timestamp as version in the index name.
 
 More complex example highlighting the possible relationships available in
 elasticsearch - parent/child and nested.
+
+``composite_agg.py``
+--------------------
+
+A helper function using the `composite aggregation
+<https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-composite-aggregation.html>`_
+to paginate over aggregation results.
+