Provide 'certfile' and 'password' parameters

IPython's auth and SSL features are a bit dinky, so we decided to keep the
service private rather than rely on the robustness of these features. I'd like
this module to be useful to third parties, though, so I added class parameters
for these configuration options. I'm sure they have their use-cases -- just not
for us.

I also added an explicit check for $::operatingsystem which fails if it's not
"Ubuntu". I'd like to add support for additional platforms in the future, but
in the meantime it's better to clearly communicate this limitation.

Change-Id: Idd4f5c6f6ac6c286d7fd661908748cd4c8d3f759