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 like this idea, and I can't think of a reason why it wouldn't work or not to do it. So I'll give it a go and add it to the PR.