Partial implementation of the top hits aggregation

TODO: Still need to implement a top hits response object and handle
deserialization in the AggregationConverter class.
Merge branch 'feature/aggregations-top-hits' of github.com:elasticsearch/elasticsearch-net into feature/aggregations-top-hits

Conflicts:
	src/Nest/DSL/Aggregations/AggregationDescriptor.cs
	src/Nest/Nest.csproj
	src/Tests/Nest.Tests.Integration/Aggregations/MetricAggregationTests.cs
added support for Top Hits aggregation
Merge branch 'develop' into feature/aggregations-top-hits
made Score backward compatible
Merge pull request #940 from elasticsearch/feature/aggregations-top-hits

Feature/aggregations top hits