Added back SimpleExample's AppDelegate, which was mistakenly overwritten...

... with the content of EncryptionExample in this commit: https://github.com/realm/realm-cocoa/commit/e4ac3011452b5f30b35d138694ae6d809cd6e514

Thanks to StanislavK for reporting in #625 !

(CHANGELOG.md not updated as it was never released.)

Review: @jpsim