Some minor updates that seem to help with Facebook

Had trouble configuring the module, so I added a some more to the readme to help configure it. I also used new versions of the api (not included in pull request). I found the scope and congif parameters on login needed to be adjusted.

I'm also wondering if teh redirects in the login callbacks should use Controller::curr()->redirect() instead of $this. I found it created problems when SilverStripe wasn't running in domain root. (for instance on mu local maching http://localhost/site/)