Added seconds to EmailQueue::getBatch()

While testing, I was not getting all my queued emails to go out. Further investigation showed that there was up a 59 second delay because EmailQueue::getBatch() did not account seconds in the conditions for fetching emails queued. For example given this:
```php
$send_at = '2013-10-06 16:14:01';
ClassRegistry::init('EmailQueue.EmailQueue')->enqueue('mail@example.com', array(), array(
    'send_at' =>$send_at,
    'config' => 'testConfig',
));
```
10 seconds later (2013-10-06 16:14:11) when EmailQueue::getBatch() is run, it will grab the emails who's `send_at` date is less than or equal to `2013-10-06 16:14`. Since the send out was set to `2013-10-06 16:14:01` it does not get fetched. The email would be sent out at `2013-10-06 16:15`.

I know it might be a little anal, but if i wanted a 1 minute delay on my email  i would simply choose so when defining the `send_at` parameter.