Add openpgpg2pem.

Add openpgpg2pem.

With the previous setup, it was difficult for me to check https keys
when signing them.  Now I can easily extract public key information
for validating keys with

  $ gpg --export 'https://www.physics.drexel.edu' \
    | openpgp2pem | openssl rsa -in /dev/stdin -pubin -text

And compare the modulus and exponent with those given for the server's
key

  $ openssl rsa -in private.pem -pubout -text

`openpgp2pem` is also useful when confirming a server's public key
through your browser's key acceptance window (without msva), as the
modulus and exponent are readily available.