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 think this should be `Object[] propertiesCounts = (Object[]) histo.getProperty("sum.value");` else the tests don't pass.