Support for 10.9's one-level-deeper OS within BaseSystem.dmg

This adds support for 10.9 by checking the `SystemVersion.plist` at the new location if it detects a 10.9 installer. The only added logic is the new path to the plist one dmg deeper, in order to check the version. The rest of the script doesn't seem to need any changes.

I've tested a built 10.9 installer working on my FV2-encrypted Mac.