Added Provider for Elastic Search

This pull requests adds support for elastic search geospatial provider, which returms geojson like solr or postgis provider already present.

layer configuration looks like below and needs elasticsearch-py driver installed from http://www.elasticsearch.org/guide/en/elasticsearch/client/python-api/current/


{
  "cache":
  {
    "name": "Test",
    "path": "/tmp/stache",
    "umask": "0000"
  },
  "layers": 
  {
   "es": {
       "provider": {"class": "TileStache.Goodies.Providers.ESGeoJSON.Provider",
                    "kwargs": {
                       "es_endpoint": "localhost:9200",
                       "es_index":"places",
					   "es_size":1000,
                       "es_record": "place.location"
                    }}
   }
  }
}