Keychain and file encryption

User credentials are no longer stored in NSUserDefaults, but in the keychain. The wallet file is encrypted. 

When a user updates to this version, his credentials and wallet are erased. The latter is not essential; I was being lazy.

I had to increase the deployment target from 3.4 to 4.3 in order to support file encryption. It's probably not a bad idea to drop support for anything below iOs 6: less code = less potential security issues.