Do initial changes in lang.py for 0.9.0 syntax. #151
Command changes for new Yaml semantics:

- Throw away most of Command methods.
- Move format_str to lang.
- Get rid of some circular imports (hopefully for good).
Fix dependency installation for new Yaml syntax
Provide some documentation for Command class
Make format_str function work again
Fix LogCommandRunner for the new Yaml syntax
Strip exec flag of command type, but remember that it was used
Make AskCommandRunner work again (and better this time)
Fix eval_input_section to assign only res, not lres to its members
Fix CallCommandRunner for the new DSL style
Workaround the cyclic import in lang, command and command_runners
Fix cl command runner for the new DSL spec.
Fix Command instantiations, since it now has one more argument
Fix DotDevassistantCommandRunner for new Yaml syntax
Fix GitHubCommandRunner with new Yaml syntax
Fix SCLCommandRunner with new Yaml syntax
Fix Jinja2Runner with new Yaml syntax
Fix DockerCommandRunner with new Yaml syntax
Fix tests in test_lang
Fix tests in test_yaml_assistant