Several bugfixes involving GnuPG interop

Hi, I've been using openpgpjs to read files coming from GnuPG, and have found a few interoperability issues, this is a pull request which fixes a few small things.

First, it adds support for a GnuPG extension, for stripping primary key secrets (described here: http://www.gnupg.org/faq/GnuPG-FAQ.html#how-can-i-use-gnupg-in-an-automated-environment). It also fixes a bug I encountered with signature verification when CAST5 is being used as the cipher.

I'd appreciate any feedback, thanks!