Children aggregation support

Closes #1061
Merge branch 'develop' into feature/aggs-children

Conflicts:
	src/Nest/DSL/Aggregations/AggregationDescriptor.cs
formatting commt
added f=>f.FullyQualified().ChildName support to expression based property lookups
Merge pull request #1132 from elasticsearch/feature/aggs-children

Children aggregation support