Add support for PuTTY PPK format #27

It would be wonderful to be able to use PuTTY's PPK format. I have a UI where users can generate SSH keys and paste them into a text area for upload to the server. The server only allows for openssh style keys, so PuTTY users are cranky about needing to do conversion.

I've generated three keys, all encrypted with the password "asdf". According to the documentation I've found, the public portion is always unencrypted. The private portion may be encrypted and the hash at the end is a hash of both the public and private portions.

I've generated these with puttygen, available as the Ubuntu package. I can generate more keys if that helps. The format seems very straightforward, with either a "Key-name: value" or "key-name-lines: count" style format.

putty-dsa.ppk.txt
putty-dsa-as-private-openssh.txt
putty-dsa-as-public-openssh.txt
putty-rsa.ppk.txt
putty-rsa1.ppk.txt