Modified `walkTree`

`walkTree` is now more generalist. It can take action in files **and**
directories. Also, depending on the type of action we want to do, we may
prefer to take the action in a directory before processing it's content
(top down), or after (bottom up).
Added a `handle clean` command.
BROKEN: Added a **utility** category, for `walkTree`

For now, walkTree is in this category. Here will come all helper
functions used throughout the code.

That commit is broken, because `walkTree` is now called before being
defined. The fix come right now ;-)
Fix: Taking the action all in one place

That fix the problem of utilities not defined, and make the code more
readable. The thing is: we do not call any code before the end of the
file.