Layout fixing

@dmytton, @bassdread, @jsoucheiron: Here you have my proposal for the new layout for the agent that would allow us to publish it to pypi repository and install it with 'pip install sd-agent'.

This new layout also allows us to write tests in a more easy way, given that everything is contained into functions or class methods so you can do a module import without actually execute code. We will need more code refactoring to write tests, but this just pretend to be an start point for it.

Even with the new layout you are still able to execute './bin/sd-agent start' locally without installing anything.