Change accessToken of FBSession class to readwrite

I came across this issue when trying to post to a Facebook Page (that the authenticated user has admin access to) instead of to the authenticated user's timeline. The `accessToken` property of `FBSession` was `readonly`, but posting to a Page requires that you send the Page's access token instead of the user's (retrieved from `/me/accounts`). By allowing it to be set, you can simply change the active session's `accessToken` to that of a Page.