Added injectable persistent data handler

This addresses one of the issues from #25.

Ideally the SDK would have some sort of Factory or service class that we could inject all the dependancies and configuration into. But since we don't have one, I added the injection setter to `FacebookSession` since it's a requirement for all Graph requests.

A developer can now inject their own persistent data handler. For example, here's a [Laravel](http://laravel.com/) implementation:

```php
use Facebook\FacebookPersistable;
use Facebook\FacebookSession;

class LaravelFacebookPersistentDataHandler implements FacebookPersistable
{
  public function setPersistentData($key, $value)
  {
    Session::put($key, $value);
  }

  public function getPersistentData($key, $default = null)
  {
    return Session::get($key, $default);
  }
}

FacebookSession::setPersistentDataHandler(new LaravelPersistentDataHandler());
```

I think it'd be nice to have any extensible features documented in the README.