[Aggregations] Added _meta construct to inject metadata

This PR adds the ability to associate a bit of state with each individual aggregation.

Use cases:
- Typed clients can inject some state so that the deserialization
  process has enough information to deserialize the aggregations into
the correct type. Facets return `_type` to signal what kind of facet the
response corresponds with.
- UI state, since aggregations can be nested at arbitrary depths
  injecting pieces of UI state can greatly simplify programming with
aggregations since there are no two tree data-structures to walk anymore only the aggregation data.

thank you @martijnvg for the help reading the `byte[]` representation of _meta properly

- this PR also fixes the response format of the new `geo_bounds` aggregation to wrap the result in a `"aggregationname" : {}`

[Here's a gist of a sense session](https://gist.github.com/Mpdreamz/785a3347d4415e1fe6bb#file-aggs-metada) you can use to review this branch


Example request:

```json
{
"aggs": {
    "name": {
      "terms": {
        "field": "title"
      },
      "meta": 2,
      "aggs": {
        "viewport" : {
          "meta" : {
               "complex" : "object"
          },
          "geo_bounds" : {
            "field" : "location"
          }
        },
        "empty_agg" : {
          "meta" : "should still be returned",
          "sum": {
            "field": "i_do_not_exist"
          }
        },
```


Example response:

```json
{
   "aggregations": {
      "name": {
         "meta": 2,
         "buckets": [
            {
               "key": "banner",
               "doc_count": 1,
               "viewport": {
                  "meta": {
                    "complex": "object"
                  },
                  "bounds": {
                     "top_left": {
                        "lat": 28,
                        "lon": 21
                     },
                     "bottom_right": {
                        "lat": 28,
                        "lon": 21
                     }
                  }
               },
               "nested_terms": {
                  "meta": {
                     "complex": "object",
                     "nuber": 1
                  },
                  "buckets": [
                     {
                        "key": "a",
                        "doc_count": 1,
                        "counter": {
                           "meta": 4.1,
                           "value": 40
                        }
                     },
```


the way that we usually do this is by making toXContent final, write the name and metadata, and delegate to a `doXContentBody` method. This ensures that no aggregation can by-pass the serialization of the metadata.