Add support for adding multiple fields to QueryStringQueryBuilder

Adds support for adding multiple fields at once to QueryStringQueryBuilder. It is already possible to do this via JSON, but the Java support is missing. 
<pre><code>
{
    "query_string" : {
        "fields" : ["content", "name"],
        "query" : "this AND that"
    }
}
</code></pre>
http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/query-dsl-query-string-query.html