Facebook sso

Hi again,

Here is an implementation of Facebook SSO. 
It has the same requirements as Facebook SDK, which are:
- the developer has to handle callback with
`- (BOOL)application:(UIApplication *)application
            openURL:(NSURL *)url
  sourceApplication:(NSString *)sourceApplication
         annotation:(id)annotation
{
    return [SimpleAuth handleCallback:url];
}`
- Facebook prerequisites (the .plist configuration) still applies as seen in [Facebook's login getting started guide](https://developers.facebook.com/docs/ios/getting-started)

It should handle most cases, but still needs to be heavily tested. 
We decided that the FBSession should be closed after each opening, so the provider will fetch a new token each time it is called.
Facebook's errors are handled and passed as underlying errors, and it is left to the developer's choice to specifically handle them.

We think that it is as simple as it can be for the developer, and that you will like it. :)

I updated the podspec to include Facebook iOS SDK, but you should review it as I'm no pro with .podspec…

Julien