Add SSLTransport and SSLClient to Bernhard

Hey @banjiewen - I've added a SSL support to Bernhard which is compatible with Riemann's SSL support!

I chose to implement it as a separate client rather than switching behaviors depending on the 'transport' selected in the base Client class, because an SSL connection needs more than host/port (cert, key, CA), which changes how the client instantiation has to be done.

I also added an example ssl_client, which uses Riemann's own client certificates, making testing against Riemann master totally seamless.