Updated release to emit build settings from Xcode 5.0 or later.

Xcode 5.0 requires an action (e.g. `archive`) to emit build settings.
Signing now uses OS X Keychain rather than stored files. Closes #1.

The option to use files and OpenSSL for signing still exists, just set `USE_KEYCHAIN` to something other than `1` and set the certificate and key file as always.