iOS uuid should remain even after uninstall

The readme states that an iOS quirk is that it is using the identifierForVendor, which remains immutable until every app by one vendor is uninstalled. Typically I'd imagine developers are making one app - and so if the single app gets uninstalled, they lose the uuid and it appears to be a "new" device on reinstall.

Instead, we should consider using the CFUUIDCreate to create a unique ID on startup, and insert it into the SSKeyChain of the device. Then when an app is installed, it should check to see if there already is a UUID in the key chain before generating a new one. Example: http://stackoverflow.com/questions/7273014/ios-unique-user-identifier/8677177#answer-8677177

If the device is flashed or reinstalled, then I guess it will still be a problem. I wish we could get a totally unique ID for that actual physical device.