HaveInDatabase will now return the primary id of the inserted row

Easy change but its very useful.

The DB Module already used `LastInsertId` to remove the entry after the test.
But i would like to use it in my test, so I return the `LastInsterId`.

Simple example:
```php
$personId = $I->haveInDatabase('person', array(
    'username' => 'Mitchel'
));

$I->sendPOST('/index', array(
    'function' => 'REMOVE-PERSON',
    'personid' => $personId
));
```

Maybe I should add a example in the documentation. First waiting for the feedback.