Aggregations: Added getProperty method to Aggregations

This allows arbitrary properties to be retrieved from an aggregation tree. The property is specified using the same syntax as the
order parameter in the terms aggregation. If a property path contians a multi-bucket aggregation the property values from each bucket will be returned in an array.
I am wondering if it makes sense to implement  `getProperty()` for `Aggregations` as well and not just for `Aggregation`. For example in a test I would write something like 

```
Aggregations agg = searchResponse.getAggregations();
Object o =agg.get("aggname").getProperty("path");
```
but if Aggregations also implemented getProperty() I would save another line.