Adds simple support for jqGrid Filtering

I've added simple support for filtering. It works. I've tested it using a custom search form and setting the filters manually via Js. Applying the filters and reloading the grid will show the filtered results.
There are some coding style issues. (I try to follow [PSR-1](http://www.php-fig.org/psr/psr-1/) & [PSR-2](http://www.php-fig.org/psr/psr-2/)).

I recommand you to use [php-cs-fixer](https://github.com/fabpot/PHP-CS-Fixer)