Add ignore_retweet option
We want to be able to filter retweets setting `ignore_retweet: true` (default to `false`).

```sh
curl -XPUT localhost:9200/_river/my_twitter_river/_meta -d '
{
    "type" : "twitter",
    "twitter" : {
        "user" : "twitter_user",
        "password" : "twitter_password",
        "ignore_retweet" : true
    }
'
```

Closes #24.