Facebook OAuth2 object, make various requests
the the graphAPI
@param {string} appId     the facebook app id
@param {string} appSecret the facebook app secret
@param {string} redirectURL  the url facebook should use as a callback
function FBOAuth2(appId, appSecret, redirectURL){
  this._appId = appId;
  this._appSecret = appSecret;
  this._redirectURL = redirectURL;
  this._authURL = 'https://www.facebook.com/dialog/oauth';
  this._graphAPIURL = 'https://graph.facebook.com';
  this._accessTokenURI = '/oauth/access_token';
  this._profileURI = '/me';
}