Expose heartbeat option
We can easily expose `heartbeat` option with default value to `30m`.

```sh
curl -XPUT 'localhost:9200/_river/my_river/_meta' -d '{
    "type" : "rabbitmq",
    "rabbitmq" : {
        // Skipped other values
        "heartbeat" : "30m"
    }
}'
```

Closes #29.
Update to elasticsearch 0.90.2