Adding support for custom types

Adding support for custom types. Sample configuration: 

```php
$options = array(
    'orm.em.options' => array(
        'types' => [
            'blob' => 'MyCorp\Persistence\Type\Bytea',
            'datetime' => 'MyCorp\Persistence\Type\Timestamp',
            'datetimetz' => 'MyCorp\Persistence\Type\TimestampWithTimeZone',
        ]
    )
);
```