Bulk scripting support to RabbitMQ river

Added bulk scripting support to RabbitMQ river:

```sh
curl -XPUT 'localhost:9200/_river/my_river/_meta' -d '{
    "type" : "rabbitmq",
    "rabbitmq" : { },
    "bulk_script_filter" : {
        "script" : "mock_script",
        "script_lang" : "native"
    }
}'
```
Have a look at [Native (Java) Scripts](http://www.elasticsearch.org/guide/reference/modules/scripting/) documentation for how to declare a script.

Relative to #17.