MultiPercolate should have PercolateMany

To provide syntactic sugar over e.g:

```
var multiPercolateResponse2 = clientPerco.MultiPercolate(mp => { 
    foreach(var article in listArticles) 
    {
        mp.Percolate<Article>(perc => perc 
            .Index( ElasticPercolatorClient.indexName ) 
            .Document(article) 
        );
    }
    return mp;
});
```
