Add support for verifying graph API calls with appsecret_proof

Facebook [recommends](https://developers.facebook.com/docs/reference/api/securing-graph-api/) adding the `appsecret_proof` parameter to all API calls to verify that the access tokens are coming from a valid app.  I've added a method to `setAppSecret` to set the app secret used to generate the `appsecret_proof` parameter for all API requests.  If both an app secret and an access token are present, the hash will be generated and sent to Facebook automatically for all API calls.