foreach support for rdbms.ResultSet

## Current semantics
Create a resultset, iterate until false is returned:

```php
$r= $conn->query('select news_id, caption, created_at from news'); 
while ($row= $r->next()) { 
  var_dump($row); 
} 
$r->close(); 
```

## Usage with `foreach`
Simply use the statement:

```php
$r= $conn->query('select news_id, caption, created_at from news'); 
foreach ($r as $row) {
  var_dump($row); 
}
```

This also makes `rdbms.ResultSet`s usable inside [data sequences](https://github.com/xp-forge/sequence)