Initial SSH server support.
Introduce the concept of auth handlers.

Somehow similar to views. We pass command and its arguments as username,
the thing splits it and looks for handler for the command.
Reset RNG in @postfork.
Support authentication using SSH.
Store SSH handler list in settings.
Add tests for SSH handlers.
Merge pull request #76 from mgorny/ssh-auth

SSH authentication support
Move OkupyError to okupy/__init__