Uniform server-side login

Till now getting a login link was done by using

```
<? FB_OAuth::link( $GLOBALS['config']['facebook']['scope'] ) ?>
```

This suffices for website logins as the OAuth authentication is more straightforward. 

For Facebook apps and tabs this is inadequate as it redirects back to the website URL. 

Extending its functionality with a custom redirect link may be a welcomed enhancement. 
