Add support for RSA PKCS 1.5 as signing protocol version 1.2 (via skeptomai)

This series of patches from @skeptomai adds support for RSA PKCS 1.5 to our signing and authentication protocol.

I've enhanced the test coverage and fixed a couple of buglets on top. This works for me when loaded into OSC and does not change the default protocol, just adds support for a new one.

Note that this is the Erlang server side of:
http://tickets.opscode.com/browse/CHEF-2381

This PR supercedes: https://github.com/opscode/chef_authn/pull/7