Added facebook api versioning

I have added version support for api calls. even though its only applied to facebook  service provider, we can apply to any service provider.

Reason for implementing this is, 
 I'm using https://github.com/artdarek/oauth-4-laravel in my current project which uses your https://packagist.org/packages/lusitanian/oauth as a dependency.

My issue was, when the system make facebook api calls I couldn't specify the version. After this patch I can configure artdarek/oauth-4-laravel to use api version. If the api version is not mentioned or empty it uses default url. This will avoid "You must upgrade this app to Graph API v2.x ..." login box error message and so on.

Thank you