Parse algorithm-specific data in _parse_generic_secret_key_packet

Parse algorithm-specific data in _parse_generic_secret_key_packet

From RFC 4880 [1]:

  Algorithm-Specific Fields for RSA secret keys:

  - multiprecision integer (MPI) of RSA secret exponent d.
  - MPI of RSA secret prime value p.
  - MPI of RSA secret prime value q (p < q).
  - MPI of u, the multiplicative inverse of p, mod q.

  Algorithm-Specific Fields for DSA secret keys:

  - MPI of DSA secret exponent x.

  Algorithm-Specific Fields for Elgamal secret keys:

  - MPI of Elgamal secret exponent x.

We'll need these if we want to sign new subkey bindings.

[1]: http://tools.ietf.org/search/rfc4880#section-5.5.3