Update base_facebook.php

Added the extra param: appsecret_proof to the oauth api calls

https://github.com/facebook/facebook-php-sdk/commit/0b0d711498fb4a283ce84802c34e49a491134a1f#src/base_facebook.php
Merge pull request #17 from maximerassi/master

Update of base_facebook.php from facebook / facebook-php-sdk