Return value of setExtendedAccessToken

I added return value to setExtendedAccessToken function to
determine of success API operation. This provides to get
correct access_token value from getAccessToken function

Correct use case of setExtendedAccessToken:

```php
if ($fb->setExtendedAccessToken()) {
    $my->storeAccessToken($fb->getUser(), $fb->getAccessToken());
}
```

I made small fix for malformed multiline comment