Get public key from private key

[robintemme]

In your jwt_backend.go you could retrieve the RSA public key part from the private key, because due to logic and the official GoDocs of rsa.PrivateKey the PublicKey is contained in the PrivateKey. This helps to avoid a lot of action.

Just a suggestion, because I use a similar backend in my current project:

// GetPublicKey returns the rsa public key from the private key
func GetPublicKey(privateKey *rsa.PrivateKey) *rsa.PublicKey {
    return &privateKey.PublicKey
}

Sorry I'm too lazy right now to create a pull request, nonetheless thanks for this project (and the regarding blog post), it helped a lot! 👍 🙇