Add a flexible packet-parsing framework

Add a flexible packet-parsing framework

Shunting packet-type processing out to type-specific methods.  For
example, 'public-key packet' packets will be parsed by
PGPPacket._parse_public_key_packet.

From the re docs [1]:

  '+'
    Causes the resulting RE to match 1 or more repetitions of the
    preceding RE. ab+ will match 'a' followed by any non-zero number
    of 'b's; it will not match just 'a'.

  ...

  \W
    For Unicode (str) patterns:
      Matches Unicode word characters; this includes most characters
      that can be part of a word in any language, as well as numbers
      and the underscore. If the ASCII flag is used, only [a-zA-Z0-9_]
      is matched (but the flag affects the entire regular expression,
      so in such cases using an explicit [a-zA-Z0-9_] may be a better
      choice).

    For 8-bit (bytes) patterns:
      Matches characters considered alphanumeric in the ASCII
      character set; this is equivalent to [a-zA-Z0-9_].

[1]: http://docs.python.org/3/library/re.html#regular-expression-syntax