Fix log-in "remember me" not functioning completely as it should

This is a fix for #46.

All we do here is delete **all** cookies from the browser that have come from UpStage instead of just the `username`, `password`, and `usernameUpStage` cookies. See [this comment](https://github.com/AUTUpStageTeam/UpStage2013/issues/46#issuecomment-24896539) on the original issue for more info.