Merge: nitdoc: introduces phases

This serie of commit introduces the concept of DocPhase.

The first commit introduces new concepts that are used by the other commits to move code.

For each phase, related code is extracted from `doc_pages` to the corresponding `doc_phases`.
No code is actually deleted from `doc_pages` between steps. The complete module is deleted at the end.

Refactoring is kept at a minimal level in this PR (just enough to get compatibility between old `doc_model` and new `doc_phases`).
Some hacks are used to preserve a minimum of differences in the generated HTML.

Further commits will continue the cleaning / refactoring but also change the outputed HTML, so I prefer wait for a new PR.
Demo (wait for Jenkins): http://gresil.org/jenkins/job/CI-nitdoc/

Pull-Request: #1135
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>