Added min_timestamp and max_timestamp to searchMedia

http://instagram.com/developer/endpoints/media/#get_media_search

An example to retrieve photos taken near a concert venue during an event:

```
$search = array(
    'lat' => '59.920016',
    'lng' => '10.7524388',
    'min_timestamp' => strtotime('2013-11-21 23:15 +0100'),
    'max_timestamp' => strtotime('2013-11-22 03:00 +0100'),
    'distance' => 100,
    'count' => 20
);
```