Implementation to support geo circles (closes #2713)

This one is based on spatial4j in order to prevent the creation of an own geo_circle type.

The GeoJSON standard does not support circles. A proposal for this has been rejected:
https://github.com/GeoJSONWG/geojson-spec/wiki/Proposal---Circles-and-Ellipses-Geoms

This implementation extends the geo_shape parsers and serializers to support circles.

It works like this:

```
curl -X DELETE localhost:9200/geotest
curl -X PUT localhost:9200/geotest
curl -X PUT localhost:9200/geotest/geotest/_mapping -d '{ "geotest": { "properties":  { "location" : { "type":"geo_shape" } } } }'

curl -X PUT localhost:9200/geotest/geotest/parkhotel?refresh=true -d '{ "name":"Park Hotel Amsterdam", "location" : { "type" : "point", "coordinates" : [4.883208, 52.362105] } }'
curl -X PUT localhost:9200/geotest/geotest/schiphol?refresh=true -d '{ "name":"Schiphol Airport", "location" : { "type" : "point", "coordinates" : [4.77253, 52.313096] } }'

echo "Querying with a radius of 10km (excluding the airport)"
curl -X POST localhost:9200/geotest/geotest/_search -d '{
  "from":"0",
  "size":"10",
  "query" : {
    "filtered" : {
      "query": { "match_all" : {} },
      "filter" : {
        "geo_shape" : {
          "location" : {
            "shape" : {
              "type" : "circle",
              "coordinates" : [ 4.895439, 52.37125 ],
              "radius": "10"
            }
          }
        }
      }
    }
  }
}'

echo "Querying with a radius of 15km (including the airport)"
curl -X POST localhost:9200/geotest/geotest/_search -d '{
  "from":"0",
  "size":"10",
  "query" : {
    "filtered" : {
      "query": { "match_all" : {} },
      "filter" : {
        "geo_shape" : {
          "location" : {
            "shape" : {
              "type" : "circle",
              "coordinates" : [ 4.895439, 52.37125 ],
              "radius": "15"
            }
          }
        }
      }
    }
  }
}'
```

TODO:
- Add real corner case tests