Adds recursion to modules parsing

[marimeireles]

This adds recursion to memestre and allows us to do stuff like:

memestra tests/misc/memestra_nb_demo.ipynb --decorator "decoratortest.deprecated"

[serge-sans-paille]

You've been killing the validation :-)

[serge-sans-paille]

You've been killing the validation :-)
Can you add a test case (or more!) that describes your intent?
Also it seems this commit is trying to do several things in a single commit, I'd rather review one commit per change.

[serge-sans-paille]

Obsoleted by #23