Add ability to dump public key

[zbyte64]

One should be able to:

dump_publickey(crypto.FILETYPE_PEM, pkey)

Resurrected from:
https://code.launchpad.net/~shanemhansen/pyopenssl/dump_publickey

[hynek]

Related code on how to do it w/ the new cryptography backend: https://github.com/hynek/tnw/blob/master/danex/_x509.py

[alex]

There's a dump_publickey method these days.