reworked GetBasePath on OS X to use Contents/Resource by default if bundled, or exedir if not bundled.

- also adds OS X specific magic for bundled apps adding an Info.plist property of name SDL_FILESYSTEM_BASE_DIR_TYPE to the following values will change the bahaviour.
* bundle -- use the bundle directory e.g. "/Applications/MyGame/Blah.app/"
* parent -- use the bundle parent directory e.g. "/Applications/MyGame/"
* resource -- use the bundle resource directory (default) e.g. "/Applications/MyGame/Blah.app/Contents/Resources/"