Clean up appIdentifier and keychain servicename to be defined only once
Provide an iOS 5 compatible C helper for getting a UUID
Update bit_appAnonID helper to make sure to get a UUID for an app installation and works through the app installation lifetime on the device

- Only get the UUID once per app runtime
- Cache the UUID in the keychain
- Gracefully fall back if some mechanisms won't work

TODO: In theory we could simply only ever use our own generated UUID and keep that via the keychain over the app lifetime instead of trying iOS 6 deviceIdentifier and alternatively the apps pathname