Added 'logout' functionality to 'oauth2' module
Added 'logout' functionality to 'oauth2' module

* the function 'accessToken.clear' removes the access token from the cache so
  that the user has to authenticate again the next data is request from the
  server.

justsocialapps/just-sdk,justsocialapps/just-sdk,justsoftware/just-sdk,justsoftware/just-sdk