Add PGPPacket._public_key_algorithms

Add PGPPacket._public_key_algorithms

From RFC 4880 [1]:

      ID           Algorithm
      --           ---------
      1          - RSA (Encrypt or Sign) [HAC]
      2          - RSA Encrypt-Only [HAC]
      3          - RSA Sign-Only [HAC]
      16         - Elgamal (Encrypt-Only) [ELGAMAL] [HAC]
      17         - DSA (Digital Signature Algorithm) [FIPS186] [HAC]
      18         - Reserved for Elliptic Curve
      19         - Reserved for ECDSA
      20         - Reserved (formerly Elgamal Encrypt or Sign)
      21         - Reserved for Diffie-Hellman (X9.42,
                   as defined for IETF-S/MIME)
      100 to 110 - Private/Experimental algorithm

[1]: http://tools.ietf.org/search/rfc4880#section-9.1