Fixing an issue with token expiration timestamp on persist/restore.

I ran into an issue when restoring an access token created during a previous run of my app.  Basically whenever the token was restored it thought it would expire 'one hour from now' instead of 'one hour from my initial creation date' - if the app was shut down for over an hour the token would expire but the app still thought it was good.  This commit fixes this by storing the absolute expiration time.

(For reference, http://code.google.com/p/qsb-mac-plugins/issues/detail?id=5#c12 is the related ticket for the app (actually plugin for an app) in question)