Add gnupg-pkcs11-scd.rb formula

gnupg-pkcs11 is a project to implement a BSD-licensed smart-card daemon
to enable the use of PKCS#11 tokens with GnuPG. gnupg-pkcs11-scd is a
drop-in replacement for the smart-card daemon (scd) shipped with the
next-generation GnuPG (gnupg-2). The daemon interfaces to smart-cards by
using RSA Security Inc. PKCS#11 Cryptographic Token Interface
(Cryptoki).

Project website: http://gnupg-pkcs11.sourceforge.net

Additional depends not in mxcl/hombrew/master: pkcs11-helper.rb