add support for re-authentication

as discussed here, https://github.com/facebook/facebook-php-sdk-v4/pull/309

adds a new base method to generate url : makeUrl()
and other  3 methods to get a login URL, re-request URL & re-authentication URL. 
I'd like to rewrite the above part like:
```php
$params += [
    'client_id' => $this->app->getId(),
    'response_type' => 'code',
    'sdk' => 'php-sdk-' . Facebook::VERSION,
];

$params = array_merge($params, [
    'state' => $state,
    'redirect_uri' => $redirectUrl,
    'scope' => implode(',', $scope)
]);
```
``client_id``, ``response_type`` and ``sdk`` are default values, but overidable by the user in ``$params`` argument.
For others, method arguments take priority, even if the user provide them in the ``$params`` argument.

PS: 2 space indentation ;)