Added an interface when extending the BaseFacebook class

An interface to be implemented when extending from the `BaseFacebook` class. This will allow for type hinting dependency injection to ensure we're getting the correct object. So we can do stuff like this:

```php
public function __construct(\BaseFacebookInterface $sdk)
{
    $this->sdk = $sdk;
}
```

And for custom implementations, they would just do this:

```php
class MyFacebook extends \BaseFacebook implements \BaseFacebookInterface {}
```

This would cross off one of the wishlist items from #177.