A few OS X fixes

* Fixed a bug where getLocalDir() would fail when opening a .tws that had been saved on a different OS to the user's.
* Now, warning messages will no longer appear on startup when a StoryIncludes path or URL is incorrect - only when building.
* Fixed the size of StorySettings on OS X.
* Altered the way story metadata is stored in the .tws. This isn't backwards-compatible with previous private betas or release candidates, by the way.