Use BulkProcessor feature

We want to use one single shared bulk processor instead of creating multiple bulkRequest instances.

New options:

```sh
$ curl -XPUT 'localhost:9200/_river/lemonde/_meta' -d '{
  "type": "rss",
  "rss": {
    "feeds" : [ {
    	"name": "lemonde",
    	"url": "http://www.lemonde.fr/rss/une.xml"
    	}
    ]
  },
  "index": {
    "index": "myindexname",
    "type": "mycontent",
    "bulk_size": 100,
    "flush_interval": "30s"
  }
}'
```

Closes #21.