Selection->page bug

- bug report? yes
- feature request? no

### Description
Paging doesn't take into account GROUP.

### Example

```
$selection = $database->table('table1')
->group('column');
```

$selection->page sets the $numOfPages to `$selection->count('')`, which is just 
`SELECT count(*) FROM table1`,
where it should be 
```
SELECT count(*)
 FROM table1 
GROUP BY column
```
