Add elasticsearch approx plugin to graylog2

Summary: This plugin for elasticsearch will let us do cool unique date histograms - for instance a count of distinct/unique students or organizations affected by a particular query over time.

Test Plan:
In chef-core:

    rake build[logging]

Then I pointed the Berksfile at my local instance and

    knife solo cook logging

That worked fine, and I observed elasticsearch restart. I then ran it again, and saw that elasticsearch did not restart and the recipe was skipped.

Then, I pointed rails at my localhost, loaded a few pages from the quest experience and then queried my local instance with the new distinct_date_histogram that's enabled now:

    35] pry(main)> Logging::Elasticsearch::Query.new.size(0).add_facet("hist1", distinct_date_histogram: {key_field: "histogram_time", value_field: "_organization_id", interval: "minute"}).execute!
=> #<Logging::Elasticsearch::Results:0x007fcee59b0698
 @response=
  {"took"=>268,
   "timed_out"=>false,
   "_shards"=>{"total"=>2, "successful"=>2, "failed"=>0},
   "hits"=>{"total"=>603, "max_score"=>1.0, "hits"=>[]},
   "facets"=>
    {"hist1"=>
      {"_type"=>"distinct_date_histogram",
       "entries"=>
        [{"time"=>1385476980000, "count"=>1, "distinct_count"=>1},
         {"time"=>1385477040000, "count"=>10, "distinct_count"=>1},
         {"time"=>1385477100000, "count"=>4, "distinct_count"=>1}]}}}>

Reviewers: koppel, john, Alex

Differential Revision: http://ph.esparklearning.com/D1995