feature request: filter and/or combining

This commit introduces orFilter()/andFilter() method to filters,
combining them in a new OR or AND filter.

```php
$filter_blue  = new Elastica_Filter_Term(array('color' => 'blue'));
$filter_green = new Elastica_Filter_Term(array('color' => 'green'));

$filter_blue_or_green = $filter_blue->orFilter($filter_green);
```

Chaining example :

```php
// (color = blue || color = green) && likes = cookies
$filter =  (new Elastica_Filter_Term(array('color' => 'blue')))
 ->orFilter(new Elastica_Filter_Term(array('color' => 'green')))
->andFilter(new Elastica_Filter_Term(array('likes' => 'cookies')))
;
```