add OS version checking to QSRequirements

Pretty small and self-explanitory.

I mainly did this so the new [File Tagging plug-in](/quicksilver/FileTagging-qsplugin) can require 10.9.

I've tested this under 10.9, by the way. It works if you specify either "10.9.0" or "10.9" as the required OS.

Once merged, I'll update the plug-in dev reference.