New feature: where_id_in() for selecting multiple records by primary key

This PR allows selecting multiple records by passing an array of primary keys.

I.e.:
```php

    <?php
    $people = ORM::for_table('person')
                ->where_id_in(array(1, 5))
                ->find_many();

    // Creates SQL:
    SELECT * FROM `widget` WHERE id IN (1,5);
```

It also supports compound primary keys:
```php

    <?php
    $people = ORM::for_table('person_profile')
                ->where_id_is(array(
                    array('person_id' => '5', 'profile_id' => 10),
                    array('person_id' => '10', 'profile_id' => 10)))
                ->find_many();

    // Creates SQL:
    SELECT * FROM `widget` WHERE (( `person_id` = '5' AND `profile_id` = '10' ) OR
                                  ( `person_id` = '10' AND `profile_id` = '10' ));
```
Closes #199