Support for PKCS#12 format and/or Keychain certificates

[Legoless]

When you create an APNs certificate, you usually add it to the Keychain. Keychain only supports exporting to PKCS#12 format, which makes it necessary to convert the certificate to .PEM format using OpenSSL. This is an annoying step and according to OpenSSL::PKCS12 Ruby already supports PKCS12 certificates. This could be added to get rid of the conversion step.

In addition, through using Security tool we could also support the certificates in Keychain.

[mattt]

If you want to implement PKCS#12 support, I would be happy to review a PR.

There are no plans to add support for certificates in the keychain.

[Legoless]

Will do ASAP, thanks.