Prefer the UUID from disk/userdefaults over the identifierForVendor

This pull request makes sure that the notifier prefers the UUID from disk/userdefaults over the identifierForVendor to prevent a unexpected change in the user id when the user upgrades to iOS 6. This makes sure the affected user count will remain correct.

All this methods to generate/fetch a UUID makes the uuid method a bit bloated, I opened issue #10 to discuss if we can drop one of the save methods and maybe clean it up somewhat more in the future.