Add Id type

Currently primary keys using `Sequence` can be mapped incorrectly from another domain object eg. `foreignid => new Types\Integer(5)` when `Sequence` is actually `Integer(11)`.

This adds an `Id` type so that domain objects can define foreign keys using `new Types\Id()` to consistently map to an existing `Sequence` property.

eg. 
```php
public function properties()
{
  return array(
    'id' => new Sequence(),
    'userid' => new Id()
  );
}
```

Thoughts @lox?