Children aggregation support

This PR adds support for the [children aggregation](http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-aggregations-bucket-children-aggregation.html#search-aggregations-bucket-children-aggregation).

One thing to note, due to https://github.com/elasticsearch/elasticsearch/issues/4081 fields on the child object have to specified with their full path.  For instance, in this [test](https://github.com/elasticsearch/elasticsearch-net/blob/feature/aggs-children/src/Tests/Nest.Tests.Integration/Aggregations/SingleBucketAggregationTests.cs#L78), we have to specify `child.childName` instead of `c => c.ChildName`. Is it worth it, or possible, to figure out a clever way to handle this in code (i.e. `c => c.ChildName.FullPath())` ?

Also, I had to modify our test data / integration setup to support parent/child relationships.  This should be reusable and may come in handy when we need to test other parent/child APIs other than this aggregation type.

Closes #1061