Aggregation methods

Introduce ElasticRecord#aggregate and #ElasticRecord#aggregations. For example, to create a terms aggregation:
```ruby
dogs = Dog.filter(breed: 'Greyhound').aggregate('popular_colors' => {'terms' => {'field' => 'color'}})
dogs.aggregations
```
http://www.elasticsearch.org/guide/en/elasticsearch/reference/1.x/search-aggregations.html

@data-axle/developers