Support PKCS#8 SHA1 fingerprints used by AWS for generated keys.

Unfortunately [AWS uses a different fingerprint function](http://blog.jbrowne.com/?p=23) for key pairs depending on whether you imported them into AWS or used their generation button. This adds support for several different fingerprint functions. A pull request for chef-metal-fog that depends on this is forthcoming.

I’ve used the `openssl_pkcs8` gem to compute the PKCS#8 representation. It has a bit of native code that calls into the OpenSSL library linked into Ruby. Hope that’s ok, let me know if it isn’t. Thanks!