Rename keychain_simple.cpp to keychain_unsecure.cpp

As requested at https://github.com/sebsauer/Charm/commit/a369a26828032b25454f4ab5a24f41e3fe55e905#commitcomment-2647005
Merge pull request #98 from sebsauer/fixKeyChainUnsecureName

Rename keychain_simple.cpp to keychain_unsecure.cpp