Added support for 'expire' (which is sent by the Facebook API when a new access token is issued).

The 'expire' field is sent when a new access token is issued. It reports how long this token remains valid. This change stores the point in time the access token will become stale. It stores it using setPersistentData('access_token_expires', value).

Note: the expire field is the number of seconds the new access token is valid. What we store however is time() + that value. We need to add the current point in time, because we loose that afterwards of course.