Refactoring - Levain Execution Model

[jmoalves]

The Levain Execution Model should be more explicit.

How are the actions loaded?
How are the commands loaded?
How are the packages resolved?

Besides, we should be using a CLI framework (Cliffy?).

[jmoalves]

Perhaps it would be easier if we had a common place to "compile" the recipes. I mean, to read the recipes, follow the dependencies, replace variables, and so on.

Running the recipes would follow these steps:

• Resolve dependencies
• Run actions that change state, like setVar, addPath, etc. (Be careful! Some "live" action can change state. Like levainShell --saveVar. But we should not execute a live action in this step)
• Replace vars (Be careful! levain.password must not appear in a log, file, console, and so on)
• Run the live actions

Splitting actions like this could cause some errors.