Add support for GnuPG extensions in private subkeys

This change adds detection of GnuPG extensions in private/experimental
S2K specifiers (type 101), and parsing of gnu-dummy (1001) (indicating
absent secret key material), and gnu-divert-to-card (2002) (indicating
key material stored on a smartcard).
Merge pull request #8 from quinot/master

Add support for GnuPG extensions in private subkeys