Merge: nitdoc: display constructors list in MClass page

Nitdoc now displays a list of available constructors for a class.

Fixes #875
Fixes #1195

Demos from [Jenkins::CI-nitdoc](http://gresil.org/jenkins/job/CI-nitdoc):
* [Standard library](http://gresil.org/jenkins/job/CI-nitdoc/ws/doc/stdlib/index.html)
* [Nit compilers and tools](http://gresil.org/jenkins/job/CI-nitdoc/ws/doc/nitc/index.html)

Pull-Request: #1341
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>