Add method to access ES aggregations data

Elasticsearch transitions from facets to aggregations.

Add a simple accessor to the aggregations data.

Example usage:

    Stretcher::Server.with_server(url) do |s|
      s.index(index).search(
        :aggs   => {
          :visits_over_time => {
            :date_histogram => {
              :field => :time, :interval => :second
            }
          }
        }
    ).aggregations

Results:

    #<Hashie::Mash visits_over_time=#<Hashie::Mash buckets=[#<Hash...