Highlighting

I added a highlighting method. It currently accepts an array which has to be structured as seen on http://www.elasticsearch.org/guide/reference/api/search/highlighting

It's a first step, I'm sure this can be made more advanced. But I still need to wrap my head around how this bundle works. ;-) I'm fairly new to PHP (coming from Java), so certain ways of coding are new to me (like the way the queries work etc...)

Usage : 
```PHP
$request->highlight(array("pre_tags" => array("<strong>"), "post_tags" => array("</strong>"), "fields" => array("content" => array("fragment_size" => 150, "number_of_fragments" => 1))));
```